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

Change-Id: I686212a283b9e0ce769b1351e3cb75e08f4b110c
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>