Rework the Kerberos Autoconf probes

Rework the Kerberos Autoconf probes

Previously, building with Kerberos support required either passing a
flag to configure giving the location of a krb5-config script, or
manually setting variables specifying the Kerberos libraries and
header path.  Replace that code with code that checks for Kerberos
libraries automatically and builds the Kerberos code if any were
found, with support for doing direct library probing if there is no
krb5-config script.

Add several platform-specific overrides directly into the configure
support, so we should be able to build out of the box on Mac OS X 10.3,
HP-UX, and AIX Kerberos with the new probes.

The Kerberos Autoconf macros are now the versions that come with
rra-c-util and are tested with multiple other packages, so both
OpenAFS and those packages will be able to benefit from further
portability improvements.

Update README for the new building instructions, documenting how to
configure the Kerberos probes if they can't automatically figure out
the location and flags for Kerberos on your system.

Change-Id: Ia35bb0dbc6b94c6b4dfe8165388447dbfcb31a29
Reviewed-on: http://gerrit.openafs.org/2026
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>