afsd: Make mountdir check kernel-specific

afsd: Make mountdir check kernel-specific

Checking if the /afs directory exists only makes sense for the kernel
afsd. The libuafs afsd does not care if the mount directory actually
exists on the machine or not, since it may not interact with the mount
directory path on the local machine at all.

So, make the mountdir check code be a new afsd function
(afsd_check_mount), and have it stat() the mount directory only in the
kernels-specific afsd.

Reviewed-on: http://gerrit.openafs.org/3980
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit fd010651194f4c9f1324ea7aa8d84426ce9827e4)

Change-Id: Ib4f4fe3a6072f9aea4683bb1ba531da8d965b156
Reviewed-on: http://gerrit.openafs.org/3983
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>