libafs/dir: Verify directory pathnames

libafs/dir: Verify directory pathnames

Provide a new routine, afs_dir_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.

Change-Id: I4b3bbb95cb49645a8ac52e6061f9e24f89924831
Reviewed-on: http://gerrit.openafs.org/5241
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>