auth: get local cell internal function

Add an internal function to retrieve the local cell
name when the auth mutex is already held. Implement
afsconf_GetLocalCell in terms of the new internal
function.

Reviewed-on: http://gerrit.openafs.org/6875
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 15cde0e40db8c99ef9af7e9f04c406ce6ea3bfeb)

Change-Id: I93e678ed7d889c4d1d33ea824625f2dfdb1707d1
Reviewed-on: http://gerrit.openafs.org/10034
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>