Linux: Simplify header file checks

Linux: Simplify header file checks

Provide an autoconf macro to perform tests for the existence of
Linux kernel header files. Use this to standardise the naming of
header file presence #defines, and to simplify the linux tests.

Change-Id: I49629679db56c5f9a39487bd78fc2e59d5da0269
Reviewed-on: http://gerrit.openafs.org/1844
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>