Fix kadmin -k with fallback or referral realm

Fix kadmin -k with fallback or referral realm

kadmin -k produces a client principal name with
krb5_sname_to_principal(), but it gets converted to a string and back
due to the signature of kadm5_init_with_skey(), which loses track of
the name type, so no canonicalization is performed.

In libkadm5clnt initialization, recognize the important subset of this
case--an empty realm indicates either fallback processing or the
referral realm--and restore the host-based name type so that the
client principal can be canonicalized against the keytab.

ticket: 9013 (new)