Report errors on listen() to the management process.

If the listen() system call were to report problems, they are now reported
back to the management process, failing the child startup (instead of
asserting). This can happen on many widely used Linux kernels if the
listen socket already has another process listening on that port with the
SO_REUSEPORT flag set.

Newer Linux kernels will report the problem at bind() time (causing error
to be reported upon management process startup).

Fixes: #2551

bin/varnishd/cache/cache_acceptor.c

@@ -631,7 +631,12 @@ ccf_start(struct cli *cli, const char * const *av, void *priv)
 				    "Kernel TCP Fast Open: sock=%d, ret=%d %s",
 				    ls->sock, i, strerror(errno));
 		}
-		AZ(listen(ls->sock, cache_param->listen_depth));
+		if (listen(ls->sock, cache_param->listen_depth)) {
+			VCLI_SetResult(cli, CLIS_CANT);
+			VCLI_Out(cli, "Listen failed on socket '%s': %s",
+			    ls->endpoint, strerror(errno));
+			return;
+		}
 		vca_tcp_opt_set(ls, 1);
 		if (cache_param->accept_filter) {
 			int i;