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.

helpers.c

@@ -127,6 +127,7 @@ void print(int action, const char *fmt, ...)
 	va_list ap;
 	const char success[] = " \e[1m[ OK ]\e[0m\n";
 	const char failure[] = " \e[7m[FAIL]\e[0m\n";
+	const char warning[] = " \e[7m[WARN]\e[0m\n";
 	const char pending[] = " \e[1m[ \\/ ]\e[0m\n";
 	const char dots[] = " .....................................................................";
 
@@ -158,6 +159,10 @@ void print(int action, const char *fmt, ...)
 		write(STDERR_FILENO, failure, sizeof(failure));
 		break;
 
+	case 2:
+		write(STDERR_FILENO, warning, sizeof(warning));
+		break;
+
 	default:
 		write(STDERR_FILENO, pending, sizeof(pending));
 		break;