autoconf: Tidy up header includes

Tidy up our list of header includes so it's easier to see what
we're testing for. The list is now one-per-line, in alphabetically
sorted order.

Note that this won't affect our use of the default include set, as
autoconf will already have tested for all of those before it starts
this CHECK_HEADERS run.

Reviewed-on: http://gerrit.openafs.org/3612
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit c8fc94fb86255a2c5cbd1613b33e32ec3a4d3b21)

Change-Id: I0b507dcd9578d1c041731f7f7923f27e33fc89df
Reviewed-on: http://gerrit.openafs.org/9411
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>