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.

Change-Id: Ie05a587951d6d5f69ae03cf3749f8c53e1eb6b62
Reviewed-on: http://gerrit.openafs.org/5800
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>