krb5_wrap: Try to guess the correct realm from the service hostname

krb5_wrap: Try to guess the correct realm from the service hostname

If we do not get a realm mapping from the krb5.conf or from the Kerberos
library try to guess it from the service hostname. The guessing of the
realm from the service hostname is already implemented in Heimdal. This
makes the behavior of smb_krb5_get_realm_from_hostname() consistent
with both MIT and Heimdal.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12554

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Andreas Schneider <asn@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>