idmap_rfc2307: Fix handling of cn realm

idmap_rfc2307: Fix handling of cn realm

When cn_realm was set, the idmap_rfc2307 module tried to determine the
realm from the AD connection struct. In case of referring to a different
domain using the ldap_domain config option, the wrong realm was used.

Since the LDAP-server case already requires having the realm in the
config, extend that to the AD case to fix the issue: Having LDAP records
with @realm in the cn, now always requires having the realm in the
config.

Now cn_realm and ldap_realm always would have to be specified together,
so replace the two options with a single "realm" option.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>