unix: giveupallcallbacks at shutdown

just try to give up callbacks at shutdown. at this point if
you're running 1.4.5 or older, you're sad anyway.
Reviewed-on: http://gerrit.openafs.org/3404
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit cee2c677d7de66a510d05978e3b41dcd5d8aca78)

Change-Id: I56e6b9e0e5f2921126a468854a1735b257e05219
Reviewed-on: http://gerrit.openafs.org/6272
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>