Linux: Restructure kernel header detection

Linux: Restructure kernel header detection

As of kernel 3.7, version.h has moved, and hence utsrelease.h was
no longer found. Loop over candidate directories and locations
within, and look for the files we're actually after.

FIXES 131525

Reviewed-on: http://gerrit.openafs.org/8761
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Laß <lass@mail.uni-paderborn.de>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 3f9d982ec25aa5f9cf32a1268d6883a9afa70b12)

Change-Id: Ia9653405195d4c5f6592960375340553de1267c5
Reviewed-on: http://gerrit.openafs.org/8804
Reviewed-by: Michael Laß <lass@mail.uni-paderborn.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>