mhd_send: use sendmsg() in POSIX-compatible way

Some OSes may have limit on number of maximum elements per single
call of sendmsg()/writev(). Do not try to send more elements than
allowed by OS.

ChangeLog

@@ -1,3 +1,7 @@
+Web 14 Apr 2021 22:20:00 MSK
+    Fixed: use sendmsg() in POSIX-compatible way, do not try to send more
+    than IOV_MAX elements per single call. -EG
+
 Sun 11 Apr 2021 15:44:00 MSK
     Updated test TLS certificates to not expired modern versions, restored
     HTTPS examples compatibility with modern browsers.

src/microhttpd/mhd_send.c

@@ -1291,6 +1291,10 @@ send_iov_nontls (struct MHD_Connection *connection,
   pre_send_setopt (connection, true, push_data);
 
   items_to_send = r_iov->cnt - r_iov->sent;
+#ifdef IOV_MAX
+  if (IOV_MAX < items_to_send)
+    items_to_send = IOV_MAX;
+#endif /* IOV_MAX */
 #ifdef HAVE_SENDMSG
   memset (&msg, 0, sizeof(struct msghdr));
   msg.msg_iov = r_iov->iov + r_iov->sent;