examples: fix compiling with MSVC

- `websocket.c`: use `Sleep()` on Windows.
  `sleep()` and `unistd.h` are not available in MSVC.

- `http2-upload.c`: use local `gettimeofday()` implementation when
  compiled with MSVC.
  (Alternate solution is to disable the trace function for MSVC.)
  Public domain code copied and adapted from libssh2:
  e973493f99/src/misc.c (L719-L743)

- silence compiler warning for deprecated `inet_addr()`.
  Also drop duplicate winsock2 include.
  ```
  curl\docs\examples\externalsocket.c(125,32): error C2220: the following warning is treated as an error [curl\bld\docs\examples\curl-example-externalsocket.vcxproj]
  curl\docs\examples\externalsocket.c(125,32): warning C4996: 'inet_addr': Use inet_pton() or InetPton() instead or define _WINSOCK_DEPRECATED_NO_WARNINGS to disable deprecated API warnings [curl\bld\docs\examples\curl-example-e
  ```
  Ref: https://github.com/curl/curl/actions/runs/9227337318/job/25389073450#step:4:95

- silence an MSVC compiler warning. This is in conflict with `checksrc`
  rules, so silence the rule in favour of the warning-free C syntax.
  ```
  curl\docs\examples\multi-legacy.c(152,1): error C2220: the following warning is treated as an error [curl\bld\docs\examples\curl-example-multi-legacy.vcxproj]
  curl\docs\examples\multi-legacy.c(152,1): warning C4706: assignment within conditional expression [curl\bld\docs\examples\curl-example-multi-legacy.vcxproj]
  ```
  Ref: https://github.com/curl/curl/actions/runs/9227337318/job/25389073450#step:4:226

- do not use `sys/time.h` and `unistd.h` in Windows builds.
  Some of these includes look unnecessary. Subject to another PR.

Cherry-picked from #13766
Closes #13771

docs/examples/10-at-a-time.c

@@ -127,7 +127,8 @@ int main(void)
     int still_alive = 1;
     curl_multi_perform(cm, &still_alive);
 
-    while((msg = curl_multi_info_read(cm, &msgs_left))) {
+    /* !checksrc! disable EQUALSNULL 1 */
+    while((msg = curl_multi_info_read(cm, &msgs_left)) != NULL) {
       if(msg->msg == CURLMSG_DONE) {
         char *url;
         CURL *e = msg->easy_handle;

docs/examples/externalsocket.c

@@ -25,13 +25,18 @@
  * Pass in a custom socket for libcurl to use.
  * </DESC>
  */
+#ifdef _WIN32
+#ifndef _WINSOCK_DEPRECATED_NO_WARNINGS
+#define _WINSOCK_DEPRECATED_NO_WARNINGS  /* for inet_addr() */
+#endif
+#endif
+
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #include <curl/curl.h>
 
 #ifdef _WIN32
-#include <winsock2.h>
 #define close closesocket
 #else
 #include <sys/types.h>        /*  socket types              */

docs/examples/http2-download.c

@@ -31,8 +31,10 @@
 #include <errno.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>

docs/examples/http2-pushinmemory.c

@@ -30,8 +30,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>

docs/examples/http2-serverpush.c

@@ -30,8 +30,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>

docs/examples/http2-upload.c

@@ -33,8 +33,10 @@
 #include <errno.h>
 
 /* somewhat unix-specific */
+#ifndef _MSC_VER
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>
@@ -49,6 +51,26 @@
 
 #define NUM_HANDLES 1000
 
+#ifdef _MSC_VER
+#define gettimeofday(a, b) my_gettimeofday((a), (b))
+int my_gettimeofday(struct timeval *tp, void *tzp)
+{
+  (void)tzp;
+  if(tp) {
+    /* Offset between 1601-01-01 and 1970-01-01 in 100 nanosec units */
+    #define _WIN32_FT_OFFSET (116444736000000000)
+    union {
+      CURL_TYPEOF_CURL_OFF_T ns100; /* time since 1 Jan 1601 in 100ns units */
+      FILETIME ft;
+    } _now;
+    GetSystemTimeAsFileTime(&_now.ft);
+    tp->tv_usec = (long)((_now.ns100 / 10) % 1000000);
+    tp->tv_sec = (long)((_now.ns100 - _WIN32_FT_OFFSET) / 10000000);
+  }
+  return 0;
+}
+#endif
+
 struct input {
   FILE *in;
   size_t bytes_read; /* count up */

docs/examples/multi-app.c

@@ -31,8 +31,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>
@@ -84,7 +86,8 @@ int main(void)
       break;
   }
   /* See how the transfers went */
-  while((msg = curl_multi_info_read(multi_handle, &msgs_left))) {
+  /* !checksrc! disable EQUALSNULL 1 */
+  while((msg = curl_multi_info_read(multi_handle, &msgs_left)) != NULL) {
     if(msg->msg == CURLMSG_DONE) {
       int idx;
 

docs/examples/multi-debugcallback.c

@@ -30,8 +30,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>

docs/examples/multi-double.c

@@ -29,8 +29,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>

docs/examples/multi-formadd.c

@@ -33,7 +33,9 @@
 
 #include <stdio.h>
 #include <string.h>
+#ifndef _WIN32
 #include <sys/time.h>
+#endif
 
 #include <curl/curl.h>
 

docs/examples/multi-legacy.c

@@ -31,8 +31,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>
@@ -147,7 +149,8 @@ int main(void)
   }
 
   /* See how the transfers went */
-  while((msg = curl_multi_info_read(multi_handle, &msgs_left))) {
+  /* !checksrc! disable EQUALSNULL 1 */
+  while((msg = curl_multi_info_read(multi_handle, &msgs_left)) != NULL) {
     if(msg->msg == CURLMSG_DONE) {
       int idx;
 

docs/examples/multi-post.c

@@ -28,7 +28,9 @@
 
 #include <stdio.h>
 #include <string.h>
+#ifndef _WIN32
 #include <sys/time.h>
+#endif
 
 #include <curl/curl.h>
 

docs/examples/multi-single.c

@@ -30,8 +30,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>

docs/examples/persistent.c

@@ -26,7 +26,10 @@
  * </DESC>
  */
 #include <stdio.h>
+#ifndef _WIN32
 #include <unistd.h>
+#endif
+
 #include <curl/curl.h>
 
 int main(void)

docs/examples/sepheaders.c

@@ -27,7 +27,9 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#ifndef _WIN32
 #include <unistd.h>
+#endif
 
 #include <curl/curl.h>
 

docs/examples/url2file.c

@@ -27,7 +27,9 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#ifndef _WIN32
 #include <unistd.h>
+#endif
 
 #include <curl/curl.h>
 

docs/examples/websocket.c

@@ -27,7 +27,13 @@
  */
 #include <stdio.h>
 #include <string.h>
+#ifdef _WIN32
+#include <windows.h>
+#define sleep(s) Sleep((DWORD)(s))
+#else
 #include <unistd.h>
+#endif
+
 #include <curl/curl.h>
 
 static int ping(CURL *curl, const char *send_payload)