Add kinit/klist -i options to use client keytab

Add kinit/klist -i options to use client keytab

In combination with -k, -i will cause kinit or klist to use the
default client keytab instead of the default acceptor keytab.  This
gives an easy way to figure out what default client keytab name is in
use and to get credentials using it.

ticket: 7216 (new)