Kill process groups in screen_stop()

Previously only the top child process was killed, killing the process
group also takes all of the child processes with it.

Closes-bug: 1271889
Change-Id: If1864cc4f1944f417ea3473d81d8b6e8e40030c2
Convert to osc

Change-Id: I06e68809816a6f5674589bcd0b69f8e7199737f0
Merge branch 'info-update' into dt-test
Merge branch 'kill-pg' into dt-test
Merge branch 'osc' into dt-test
Merge branch 'samples-update' into dt-test
Merge branch 'cleaner' into dt-test