core: let user define start-/stop-timeout behaviour

The usual behaviour when a timeout expires is to terminate/kill the
service. This is what user usually want in production systems. To debug
services that fail to start/stop (especially sporadic failures) it
might be necessary to trigger the watchdog machinery and write core
dumps, though. Likewise, it is usually just a waste of time to
gracefully stop a stuck service. Instead it might save time to go
directly into kill mode.

This commit adds two new options to services: TimeoutStartFailureMode=
and TimeoutStopFailureMode=. Both take the same values and tweak the
behavior of systemd when a start/stop timeout expires:

 * 'terminate': is the default behaviour as it has always been,
 * 'abort': triggers the watchdog machinery and will send SIGABRT
   (unless WatchdogSignal was changed) and
 * 'kill' will directly send SIGKILL.

To handle the stop failure mode in stop-post state too a new
final-watchdog state needs to be introduced.

src/basic/unit-def.h

@@ -127,6 +127,7 @@ typedef enum ServiceState {
         SERVICE_STOP_SIGTERM,
         SERVICE_STOP_SIGKILL,
         SERVICE_STOP_POST,
+        SERVICE_FINAL_WATCHDOG,    /* In case the STOP_POST executable needs to be aborted. */
         SERVICE_FINAL_SIGTERM,     /* In case the STOP_POST executable hangs, we shoot that down, too */
         SERVICE_FINAL_SIGKILL,
         SERVICE_FAILED,