Use a lock file and flock() to serialize server start, avoids problems

when running a bunch of tmux from cron at the same time. Based on a diff
from Tim Ruehsen.

server.c

@@ -104,7 +104,7 @@ server_create_socket(void)
 
 /* Fork new server. */
 int
-server_start(void)
+server_start(int lockfd, char *lockfile)
 {
 	struct window_pane	*wp;
 	int	 		 pair[2];
@@ -161,6 +161,10 @@ server_start(void)
 	server_fd = server_create_socket();
 	server_client_create(pair[1]);
 
+	unlink(lockfile);
+	xfree(lockfile);
+	close(lockfd);
+
 	if (access(SYSTEM_CFG, R_OK) == 0)
 		load_cfg(SYSTEM_CFG, NULL, &cfg_causes);
 	else if (errno != ENOENT) {