service: handle abort stops with dedicated timeout

When shooting down a service with SIGABRT the user might want to have a
much longer stop timeout than on regular stops/shutdowns. Especially in
the face of short stop timeouts the time might not be sufficient to
write huge core dumps before the service is killed.

This commit adds a dedicated (Default)TimeoutAbortSec= timer that is
used when stopping a service via SIGABRT. In all other cases the
existing TimeoutStopSec= is used. The timer value is unset by default
to skip the special handling and use TimeoutStopSec= for state
'stop-watchdog' to keep the old behaviour.

If the service is in state 'stop-watchdog' and the service should be
stopped explicitly we still go to 'stop-sigterm' and re-apply the usual
TimeoutStopSec= timeout.

test/fuzz/fuzz-unit-file/directives.service

@@ -227,6 +227,7 @@ TimeoutIdleSec=
 TimeoutSec=
 TimeoutStartSec=
 TimeoutStopSec=
+TimeoutAbortSec=
 Transparent=
 TriggerLimitBurst=
 TriggerLimitIntervalSec=
@@ -686,6 +687,7 @@ DefaultTasksAccounting=
 DefaultTasksMax=
 DefaultTimeoutStartSec=
 DefaultTimeoutStopSec=
+DefaultTimeoutAbortSec=
 DefaultTimerAccuracySec=
 DumpCore=
 HibernateMode=