core: introduce SuccessAction= as unit file property

core: introduce SuccessAction= as unit file property

SuccessAction= is similar to FailureAction= but declares what to do on
success of a unit, rather than on failure. This is useful for running
commands in qemu/nspawn images, that shall power down on completion. We
frequently see "ExecStopPost=/usr/bin/systemctl poweroff" or so in unit
files like this. Offer a simple, more declarative alternative for this.

While we are at it, hook up failure action with unit_dump() and
transient units too.