fssync-debug: exec DAFS version if DAFS detected

fssync-debug: exec DAFS version if DAFS detected

If the user requests something that differs depending on whether the
server is DAFS or not, try to exec the DAFS-enabled fssync-debug
(dafssync-debug) for them.

Based on a conversation with Tom Keiser.

Reviewed-on: http://gerrit.openafs.org/2480
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 1466cd755ce3b48d312fb4404f494934840e586b)

Change-Id: Id87ef70853212f41d2eabbf02bf67e216d9eb43d
Reviewed-on: http://gerrit.openafs.org/4110
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>