Move cgroup dbus requests to the child to avoid a race where a spawned child

that quickly forks will have only the parent process moved to the newly created
cgroup. From Daniel De Graaf, GitHub issue 4435.

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