auth: failback to afs3-vlserver for afs3-prserver

auth: failback to afs3-vlserver for afs3-prserver

If the DNS SRV lookup is for afs3-prserver or afs3-kaserver,
fallback to a lookup for afs3-vlserver since those services
are traditionally hosted on the same machine as the vlserver.

FIXES 129887

Reviewed-on: http://gerrit.openafs.org/4676
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 19f4fa2b4870828269e0751a4874d9ce67a1bb3a)

Change-Id: I9d0e2f0ca9d4ddc5ebf9defb44c62f6bf00955d6
Reviewed-on: http://gerrit.openafs.org/4681
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>