Move cgroup dbus requests to the child

This avoids a race where a spawned child that quickly forks will have
only the parent process moved to the newly created cgroup, leaving the
early children in tmux's own cgroup.  It also avoids problems if the
spawned process inspects or changes its own cgroup.

compat.h

@@ -450,7 +450,7 @@ void		*recallocarray(void *, size_t, size_t, size_t);
 /* systemd.c */
 int		 systemd_activated(void);
 int		 systemd_create_socket(int, char **);
-int		 systemd_move_pid_to_new_cgroup(pid_t, char **);
+int		 systemd_move_to_new_cgroup(char **);
 #endif
 
 #ifdef HAVE_UTF8PROC