butc: clean xbsa shutdown on control C

when we catch a control-c, do a clean shutdown

Reviewed-on: http://gerrit.openafs.org/8141
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit aea3c71e59ec30e84ca4e7383fd3b566ce94bbb6)

Change-Id: I89665c29a522909e126214abaec008a18acb59d7
Reviewed-on: http://gerrit.openafs.org/9495
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>