DAFS: Allow non-fileserver to schedule salvages

DAFS: Allow non-fileserver to schedule salvages

Allow non-fileserver programs to schedule salvages through the fileserver
via FSSYNC (VOL_FORCE_ERROR with the FSYNC_SALVAGE reason code). Also
make the volserver schedule salvages this way when it encounters the
appropriate errors.

FIXES 124484

Change-Id: I03ecf6302436c35fec705cd6c84a40b7cdbf6f97
Reviewed-on: http://gerrit.openafs.org/787
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>