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>

plugins/pidfile.c

@@ -143,6 +143,7 @@ static void pidfile_update_conds(char *dir, char *name, uint32_t mask)
 
 				/* Complement log in service.c for non-forking services */
 				logit(LOG_CONSOLE | LOG_NOTICE, "Started %s[%d]", svc_ident(svc, NULL, 0), pid);
+				cgroup_move_svc(svc);
 			}
 		}