s4:gensec_gssapi: Use smb_krb5_get_realm_from_hostname()

s4:gensec_gssapi: Use smb_krb5_get_realm_from_hostname()

With credentials for administrator@FOREST1.EXAMPLE.COM
this patch changes the target_principal for
the ldap service of host dc2.forest2.example.com
from

  ldap/dc2.forest2.example.com@FOREST1.EXAMPLE.COM

to

  ldap/dc2.forest2.example.com@FOREST2.EXAMPLE.COM

Typically ldap/dc2.forest2.example.com@FOREST1.EXAMPLE.COM
should be used in order to allow the KDC of FOREST1.EXAMPLE.COM
to generate a referral ticket for
krbtgt/FOREST2.EXAMPLE.COM@FOREST1.EXAMPLE.COM.

The problem is that KDCs only return such referral tickets
if there's a forest trust between FOREST1.EXAMPLE.COM
and FOREST2.EXAMPLE.COM. If there's only an external domain
trust between FOREST1.EXAMPLE.COM and FOREST2.EXAMPLE.COM
the KDC of FOREST1.EXAMPLE.COM will respond with S_PRINCIPAL_UNKNOWN
when being asked for ldap/dc2.forest2.example.com@FOREST1.EXAMPLE.COM.

In the case of an external trust the client can still ask
explicitly for krbtgt/FOREST2.EXAMPLE.COM@FOREST1.EXAMPLE.COM
and the KDC of FOREST1.EXAMPLE.COM will generate it.

From there the client can use the
krbtgt/FOREST2.EXAMPLE.COM@FOREST1.EXAMPLE.COM
ticket and ask a KDC of FOREST2.EXAMPLE.COM for a
service ticket for ldap/dc2.forest2.example.com@FOREST2.EXAMPLE.COM.

With Heimdal we'll get the fallback on S_PRINCIPAL_UNKNOWN behavior
when we pass ldap/dc2.forest2.example.com@FOREST2.EXAMPLE.COM as
target principal. As _krb5_get_cred_kdc_any() first calls
get_cred_kdc_referral() (which always starts with the client realm)
and falls back to get_cred_kdc_capath() (which starts with the given realm).

MIT krb5 only tries the given realm of the target principal,
if we want to autodetect support for transitive forest trusts,
we'll have to do the fallback ourself.

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>