RedHat: Find krb5-config in /usr/bin

RedHat: Find krb5-config in /usr/bin

In RHEL6, krb5-config is no longer in /usr/kerberos/bin/, but is in
just /usr/bin/. So, try to use /usr/bin/krb5-config if we cannot find
/usr/kerberos/bin/krb5-config and krb5config was not specified.

This is a 1.4-only change, as the 'configure' on more modern branches
should have the ability to find krb5-config automatically.

Reviewed-on: http://gerrit.openafs.org/2646
Tested-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit c2eba9d57adc79b05165fc8aebaf751ea89282f5)
Change-Id: I46c6f0e3d7c2ce427eacfbb82366a7da3156d9bd
Reviewed-on: http://gerrit.openafs.org/3514
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>