flush_ports: flush POSIX message queues properly

On Linux, read() on a message queue descriptor returns the message
queue statistics, not the actual message queue data.  We need to use
mq_receive() to drain the queues instead.

Fixes a problem where a POSIX message queue socket unit with messages
in the queue at shutdown time could result in a hang on reboot/shutdown.

src/core/socket.c

@@ -2188,8 +2188,12 @@ static void flush_ports(Socket *s) {
                 if (p->fd < 0)
                         continue;
 
-                (void) flush_accept(p->fd);
-                (void) flush_fd(p->fd);
+                if (p->type == SOCKET_MQUEUE)
+                        (void) flush_mqueue(p->fd);
+                else {
+                        (void) flush_accept(p->fd);
+                        (void) flush_fd(p->fd);
+                }
         }
 }