unix: reset errno when using sendfile emulation

A common way to check if a uv_fs_t request failed is to check that
req->errorno != 0.

With uv_fs_sendfile(), when the sendfile() syscall fails, req->errorno is set
to (for example) ENOTSOCK, even when the emulation code path succeeds.

Zero errno before the call to uv__fs_sendfile_emul() to prevent that from
happening.

src/unix/fs.c

@@ -381,6 +381,7 @@ static ssize_t uv__fs_sendfile(uv_fs_t* req) {
         errno == EIO ||
         errno == ENOTSOCK ||
         errno == EXDEV) {
+      errno = 0;
       return uv__fs_sendfile_emul(req);
     }
 
@@ -412,6 +413,7 @@ static ssize_t uv__fs_sendfile(uv_fs_t* req) {
         errno == EIO ||
         errno == ENOTSOCK ||
         errno == EXDEV) {
+      errno = 0;
       return uv__fs_sendfile_emul(req);
     }