Add remain:yes option for run/task oneshot commands

Similar to systemd's RemainAfterExit=yes.  Prevents the task from
re-running on runlevel re-entry and ensures the post: script runs
when explicitly stopped or when leaving valid runlevels.

Useful for tasks that set up persistent state like firewall rules:

    task [2345] remain:yes \
         post:/usr/sbin/teardown-firewall \
         /usr/sbin/setup-firewall -- Firewall setup

Not supported for bootstrap-only tasks (runlevel S only) since these
are deleted immediately after completion.

src/svc.c

@@ -344,6 +344,10 @@ svc_t *svc_stop_completed(void)
 	for (svc = svc_iterator(&iter, 1); svc; svc = svc_iterator(&iter, 0)) {
 		if (svc->state == SVC_STOPPING_STATE && svc->pid > 1)
 			return svc;
+
+		/* Also wait for remain tasks running their post script */
+		if (svc_is_remain(svc) && svc->state == SVC_TEARDOWN_STATE && svc->pid > 1)
+			return svc;
 	}
 
 	return NULL;