listen_for_commands: stop leaking the control socket to forked procs

All the "exec"d processes would get a copy of the control socket
descriptor, because of the default UNIX semantics of fd inherit across
execv().  This was easily seen on my system where an 'lsof' revealed
that all my terminals and shells had a copy.

To fix this we add the SOCK_CLOEXEC flag whilst opening the listener
socket(), avoiding this problem.  The "bar" descriptor already handles
this by setting O_CLOEXEC in its FIFO open() call.

communications.c

@@ -52,7 +52,7 @@ listen_for_commands(void)
 	struct sockaddr_un sun;
 
 	if ((rp_glob_screen.control_socket_fd = socket(AF_UNIX,
-	    SOCK_STREAM | SOCK_NONBLOCK, 0)) == -1)
+	    SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0)) == -1)
 		err(1, "socket");
 
 	if (strlen(rp_glob_screen.control_socket_path) >= sizeof(sun.sun_path))