Robustify service shutdown

* Save PID when using screen in screen_it()
* Add screen_stop()
* Call out service stop_*() in unstack.sh functions so screen_stop()
  can do its thing

Closes bug 1183449

Change-Id: Iac84231cfda960c4197de5b6e8ba6eb19225169a
Merge branch 'cleaner' into dt-test
Merge branch 'screen-stop' into dt-test