auth: Rework PickClientSecObj

auth: Rework PickClientSecObj

When called in localauth mode, this function was using
afsconf_GetLatestKey to check that the machine has a key file. However,
the ClientAuth and ClientAuthSecure functions then go on to do exactly
the same thing.

Instead, pick up on ClientAuth returning a rxnull security layer, and
trigger the NOTFOUND error based on that, rather than on the absence of
an old-style key file.

Change-Id: Ifb2d3a98bca5d1d67e303dcfeab1bb6f1efdc570
Reviewed-on: http://gerrit.openafs.org/4053
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>