autoconf: refactor linux-checks.m4

autoconf: refactor linux-checks.m4

Further refactoring of the autoconf macros. Divy up the linux kernel
checks into smaller files.

This is a non-functional change. Care has been taken preserve the
ordering of the autoconf tests. Except for whitespace, the generated
configure file has not been changed by this refactoring.  This has been
verified with a 'diff -u -w -B' comparison of the generated configure
file before and after applying this commit.

Change-Id: I5ea4c9e3a0aeff1767ef561bdb8361781694ee28
Reviewed-on: https://gerrit.openafs.org/12844
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>