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.