units: make ExecStopPost action part of ExecStart

Currently after exiting rescue shell we isolate default target. User
might want to isolate to some other target than default one. However
issuing systemctl isolate command to desired target would bring system
to default target as a consequence of running ExecStopPost action.

Having common ancestor for rescue shell and possible followup systemctl
default command should fix this. If user exits rescue shell we will
proceed with isolating default target, otherwise, on manual isolate,
parent shell process is terminated and we don't isolate default target,
but target chosen by user.

Suggested-by: Michal Schmidt <mschmidt@redhat.com>
(cherry picked from commit d3381512)