Use FD_CLOEXEC on ListenSockets

It's good hygiene if e.g. an extension launches a subprogram when
being loaded. We went through some effort to close them in the child
process in EXEC_BACKEND mode, but it's better to not hand them down to
the child process in the first place. We still need to close them
after fork when !EXEC_BACKEND, but it's a little simpler.

In the passing, LOG a message if closing the client connection or
listen socket fails. Shouldn't happen, but if it does, would be nice
to know.

Reviewed-by: Tristan Partin, Andres Freund, Thomas Munro
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi

src/backend/libpq/pqcomm.c

@@ -458,6 +458,9 @@ StreamServerPort(int family, const char *hostName, unsigned short portNumber,
 		}
 
 #ifndef WIN32
+		/* Don't give the listen socket to any subprograms we execute. */
+		if (fcntl(fd, F_SETFD, FD_CLOEXEC) < 0)
+			elog(FATAL, "fcntl(F_SETFD) failed on socket: %m");
 
 		/*
 		 * Without the SO_REUSEADDR flag, a new postmaster can't be started
@@ -831,7 +834,8 @@ StreamConnection(pgsocket server_fd, Port *port)
 void
 StreamClose(pgsocket sock)
 {
-	closesocket(sock);
+	if (closesocket(sock) != 0)
+		elog(LOG, "could not close client or listen socket: %m");
 }
 
 /*