auth: Permit NULL fallback in localauth case

Allow the caller of afsconf_PickClientSecObj to specify both
local authentication, and to request fallback to null authentication
if local auth isn't available.

Change-Id: Ib3b801bfde3f32b4cb9cdc7c08a3d8ab985c080f
Reviewed-on: http://gerrit.openafs.org/9022
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>