Allow forking sysv/services like podman to move between cgroups

The container monitor that podman forks off when starting a container
instance creates subgroups in the cgroup v2 hieararchy that we want to
reuse.  This patch adds cgroup_move_svc() which we call from the pidfile
plugins to relocate the conmon process.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/cgroup.h

@@ -53,4 +53,7 @@ char *cgroup_svc_name(svc_t *svc, char *buf, size_t len);
 int   cgroup_prepare (svc_t *svc, const char *name);
 int   cgroup_watch   (const char *group, const char *name);
 
+int   cgroup_move_pid(const char *group, const char *name, int pid, int delegate);
+int   cgroup_move_svc(svc_t *svc);
+
 #endif /* FINIT_CGROUP_H_ */