Clean up zombie processes on backup failure

Ensure that a SIGTERM is sent to the session leader of the process
group, so that all backup processes are terminated in case of a
FAILURE. This ensures that no zombie processes are left behind.

Closes bug 1253850

Change-Id: I4bd53c37923f83abe4c51deb95d740656ab21979
Merge "Clean up zombie processes on backup failure"