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.

Change-Id: Ic0b524e23f518c4f3c9954e6b9614bca984306a3
Reviewed-on: http://gerrit.openafs.org/3980
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>