Refactor, clean up removed top-level cgroups

The main purpose of this patch is to remove top-level cgroups when they
are remvoed from finit.conf.  However, this is tricky, becasue there may
still be supervised services (and children of them) running.  We must
postpone removal until a cgroup.events change.  To make matters worse,
events may arrive out-of-order, i.e. the last-child event for the parent
cgroup before the sub-group.

A spin-off from this patch is that we can now support arbitrarily long
group names and group config.  (Only applies to top-level cgroups, not
run/task/services.)

Limitations:
 - Processes in "removed" cgroups are not migrated/orphaned, the reasons
   are several: overhead and complexity and the fact that memory cannot
   be migrated
 - A lingering child of a moved service may prevent an old top-level
   cgroup from being removed.

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

src/cgroup.h

@@ -31,8 +31,14 @@ struct cgroup {
 	char cfg[128];
 };
 
+void cgroup_mark_all(void);
+void cgroup_cleanup (void);
+
+int  cgroup_add     (char *name, char *cfg, int protected);
+int  cgroup_del     (char *dir);
+void cgroup_config  (void);
+
 void cgroup_init    (uev_ctx_t *ctx);
-void cgroup_config  (size_t num, struct cgroup cg[]);
 
 int  cgroup_user    (char *name, int pid);
 int  cgroup_service (char *name, int pid, struct cgroup *cg);