Add housekeeping functions to clean up unused cgroups

This should not be needed, but for some reason we don't get events when
early processes exit, so we end up with lingering cgroups.

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

src/cgroup.h

@@ -56,4 +56,6 @@ 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);
 
+void  cgroup_prune   (void);
+
 #endif /* FINIT_CGROUP_H_ */