DAFS: Avoid SALVSYNC communication during shutdown

DAFS: Avoid SALVSYNC communication during shutdown

Avoid trying to contact the salvageserver for any reason while we are
shutting down. During shutdown the salvageserver may not be around
anymore, so any SALVSYNC communication will appear to hang.

Just set a global flag to indicate 'no-SALVSYNC' on shutdown, in
addition to the thread-local flag we already have.

Change-Id: Id96f20917fd0753b4934a0377b91032b2ea3d0dc
Reviewed-on: http://gerrit.openafs.org/765
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>