salvager: Implement AskDAFS via SYNC flags

salvager: Implement AskDAFS via SYNC flags

Instead of probing the DAFS-ness of the fileserver by probing which
FSSYNC opcodes it supports, detect DAFS-ness by looking at the SYNC
response header flags, which explicitly state whether or not the
endpoint is DAFS. This avoids unnecessary "protocol mismatch" log
messages when the endpoint is not DAFS.

Reviewed-on: http://gerrit.openafs.org/5800
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 7fee28bae30c5a65898a2e3fd7149f07e8ca0df6)

Change-Id: I0643b0e61795f39b63783c937e88c4a74c3c8312
Reviewed-on: http://gerrit.openafs.org/6083
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>