libafs/dir: Verify directory pathnames

libafs/dir: Verify directory pathnames

Provide a new routine, GetVerifiedBlob() which will ensure that the
pathname contained within a directory blob is correctly terminated
before returning it to the caller. For the purposes of this function,
correct termination is defined as having a terminating \0 character
within the same directory page as the blob itself.
(cherry picked from commit d1946ffe9be0031a2daf907f5e96cf0ee7f5e15e)

Change-Id: I69b9465f02417babf9b1d5179197278fac64f192
Reviewed-on: http://gerrit.openafs.org/5249
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>