service: kill stubborn services, wait before restart

Add support for registering callbacks to be called after a given
timeout.

Use this to implement forceful termination of services that refuse to
terminate upon receiving a SIGTERM. If the service PID is not collected
within 3 seconds, send a SIGKILL to it.

Also, rework restarting of crashing services. When a service crashes for
the first time, try to restart it immediately. If it crashes again, wait
a few seconds before trying again. Just as before, if it continues to
crash, block the service.

svc.c

@@ -443,6 +443,9 @@ char *svc_status(svc_t *svc)
 
 		case SVC_BLOCK_INETD_BUSY:
 			return "busy";
+
+		case SVC_BLOCK_RESTARTING:
+			return "restart";
 		}
 
 	case SVC_DONE_STATE: