socket: use accept4 when available

Linux, *BSD, and Solaris support accept4 system call that enables the
caller to assign additional flags and save some extra system calls. It
can come in handy when O_NONBLOCK or/and FD_CLOEXEC is/are required on a
socket after being accepted.

Ref:
https://man7.org/linux/man-pages/man2/accept.2.html
https://man.freebsd.org/cgi/man.cgi?query=accept4
https://man.dragonflybsd.org/?command=accept&section=2
https://man.openbsd.org/accept.2
https://man.netbsd.org/accept.2
https://docs.oracle.com/cd/E88353_01/html/E37843/accept4-3c.html
https://www.gnu.org/software/gnulib/manual/html_node/accept4.html

Closes #16979

lib/cf-socket.c

@@ -2144,7 +2144,12 @@ static CURLcode cf_tcp_accept_connect(struct Curl_cfilter *cf,
 
   if(0 == getsockname(ctx->sock, (struct sockaddr *) &add, &size)) {
     size = sizeof(add);
+#ifdef HAVE_ACCEPT4
+    s_accepted = accept4(ctx->sock, (struct sockaddr *) &add, &size,
+                         SOCK_NONBLOCK | SOCK_CLOEXEC);
+#else
     s_accepted = accept(ctx->sock, (struct sockaddr *) &add, &size);
+#endif
   }
 
   if(CURL_SOCKET_BAD == s_accepted) {
@@ -2153,7 +2158,9 @@ static CURLcode cf_tcp_accept_connect(struct Curl_cfilter *cf,
   }
 
   infof(data, "Connection accepted from server");
+#ifndef HAVE_ACCEPT4
   (void)curlx_nonblock(s_accepted, TRUE); /* enable non-blocking */
+#endif
   /* Replace any filter on SECONDARY with one listening on this socket */
   ctx->listening = FALSE;
   ctx->accepted = TRUE;