Faster return on socket error (socket close, broken connection)

When using poll() with only one file descriptor (as done in most of the civetweb code),
poll for errors as well and return -1.

This follows the same idea as in this proposed patch:
https://github.com/civetweb/civetweb/issues/993#issuecomment-822379809
but will work for all operating systems, regardless if they have poll or not.

src/civetweb.c

@@ -5912,6 +5912,13 @@ mg_poll(struct mg_pollfd *pfd,
 	 * of having to wait for a long socket timeout. */
 	int ms_now = SOCKET_TIMEOUT_QUANTUM; /* Sleep quantum in ms */
 
+	int check_pollerr = 0;
+	if ((n == 1) && ((pfd[0].events & POLLERR) == 0)) {
+		/* If we wait for only one file descriptor, wait on error as well */
+		pfd[0].events |= POLLERR;
+		check_pollerr = 1;
+	}
+
 	do {
 		int result;
 
@@ -5928,6 +5935,12 @@ mg_poll(struct mg_pollfd *pfd,
 		if (result != 0) {
 			/* Poll returned either success (1) or error (-1).
 			 * Forward both to the caller. */
+			if ((check_pollerr)
+			    && ((pfd[0].revents & (POLLIN | POLLOUT | POLLERR))
+			        == POLLERR)) {
+				/* One and only file descriptor returned error */
+				return -1;
+			}
 			return result;
 		}