Probe directly for com_err.h

com_err.h can be in com_err.h, et/com_err.h, or krb5/com_err.h (for
netbsd 6.1 and possibly other netbsd). aklog currently only includes
either com_err.h or et/com_err.h, depending on autoconf probes
performed by the krb5.m4 macros.

So, also look for krb5/com_err.h. The krb5.m4 macros currently only
look for com_err.h at all if certain other libkrb5 tests return
certain results, so just look for all of them directly in some of our
openafs-specific krb5 probing logic in configure.ac.

Also remove the duplicate check for et/com_err.h in acinclude.m4 while
we're here. We only use et/com_err.h if krb5 support is enabled, so
only check for it in the second of krb5 probes.

FIXES 131716

Reviewed-on: http://gerrit.openafs.org/10244
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 8e32b11be6ce0517f75588360a3ca8c333358ca3)

Change-Id: I567959b6d26ece9606086f4d82c5be40b11d7ea8
Reviewed-on: http://gerrit.openafs.org/10275
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>