UKERNEL: Add uafs_setMountDir

UKERNEL: Add uafs_setMountDir

Replace the function uafs_mountWithDir with uafs_setMountDir, and
adjust the one caller. This allows libuafs users to manually set the
mount dir after e.g. the mount dir is set from afsd options.

Reviewed-on: http://gerrit.openafs.org/3978
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 445548636f4d57aaa8e29912803e242f772959c7)

Change-Id: I86607ac80f564950b544a9283794f842d0dadadb
Reviewed-on: http://gerrit.openafs.org/4085
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>