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

Change-Id: Ic454b9bf7043f91654dcd1c262ab3790bf2ad272
Reviewed-on: http://gerrit.openafs.org/10244
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>