zos: add strnlen() implementation

Add an implementation of strnlen() which is not provided by default.

PR-URL: https://github.com/libuv/libuv/pull/1631
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/unix/os390-syscalls.c

@@ -427,3 +427,12 @@ ssize_t os390_readlink(const char* path, char* buf, size_t len) {
 
   return rlen;
 }
+
+
+size_t strnlen(const char* str, size_t maxlen) {
+  void* p = memchr(str, 0, maxlen);
+  if (p == NULL)
+    return maxlen;
+  else
+    return p - str;
+}

src/unix/os390-syscalls.h

@@ -66,5 +66,6 @@ int scandir(const char* maindir, struct dirent*** namelist,
             const struct dirent **));
 char *mkdtemp(char* path);
 ssize_t os390_readlink(const char* path, char* buf, size_t len);
+size_t strnlen(const char* str, size_t maxlen);
 
 #endif /* UV_OS390_SYSCALL_H_ */