autoconf: Tidy up header includes

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.

Change-Id: Iffc8c3bc7cbd56d87508ebdde38e61f8747d0501
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>