When sockd transfers a file descriptor to ndhc, close the fd in sockd.

Since the transfer is conceptually a move, this is the correct thing to
do and prevents sockets from spuriously hanging around forever and
eventually exhausting the per process limit on fds.

src/sockd.c

@@ -439,6 +439,7 @@ static void xfer_fd(int fd, char cmd)
         suicide("%s: (%s) sendmsg failed: %s", client_config.interface,
                 __func__, strerror(errno));
     }
+    close(fd);
 }
 
 static size_t execute_sockd(char *buf, size_t buflen)