Unix afsd: Check for mountpoint /afs first

Unix afsd: Check for mountpoint /afs first

Check for the existance of the mountpoint before trying to
start.

Reviewed-on: http://gerrit.openafs.org/3392
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit cc54175e43ce14c01f9c103541ffad795b17629c)

Change-Id: Ic94f1701f5d1d0d080e5751093a685e89d892b1a
Reviewed-on: http://gerrit.openafs.org/3782
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>