lookup_name: allow lookup names prefixed with DNS forest root for FreeIPA DC

lookup_name: allow lookup names prefixed with DNS forest root for FreeIPA DC

In FreeIPA deployment with active Global Catalog service, when a two-way
trust to Active Directory forest is established, Windows systems can
look up FreeIPA users and groups. When using a security tab in Windows
Explorer on AD side, a lookup over a trusted forest might come as
realm\name instead of NetBIOS domain name:

--------------------------------------------------------------------
[2020/01/13 11:12:39.859134,  1, pid=33253, effective(1732401004, 1732401004), real(1732401004, 0), class=rpc_parse] ../../librpc/ndr/ndr.c:471(ndr_print_function_debug)
       lsa_LookupNames3: struct lsa_LookupNames3
          in: struct lsa_LookupNames3
              handle                   : *
                  handle: struct policy_handle
                      handle_type              : 0x00000000 (0)
                      uuid                     : 0000000e-0000-0000-1c5e-a750e5810000
              num_names                : 0x00000001 (1)
              names: ARRAY(1)
                  names: struct lsa_String
                      length                   : 0x001e (30)
                      size                     : 0x0020 (32)
                      string                   : *
                          string                   : 'ipa.test\admins'
              sids                     : *
                  sids: struct lsa_TransSidArray3
                      count                    : 0x00000000 (0)
                      sids                     : NULL
              level                    : LSA_LOOKUP_NAMES_UPLEVEL_TRUSTS_ONLY2 (6)
              count                    : *
                  count                    : 0x00000000 (0)
              lookup_options           : LSA_LOOKUP_OPTION_SEARCH_ISOLATED_NAMES (0)
              client_revision          : LSA_CLIENT_REVISION_2 (2)
--------------------------------------------------------------------

If we are running as a DC and PASSDB supports returning domain info
(pdb_get_domain_info() returns a valid structure), check domain of the
name in lookup_name() against DNS forest name and allow the request to
be done against the primary domain. This corresponds to FreeIPA's use of
Samba as a DC. For normal domain members a realm-based lookup falls back
to a lookup over to its own domain controller with the help of winbindd.

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Alexander Bokovoy <ab@samba.org>
Autobuild-Date(master): Wed Nov 11 10:59:01 UTC 2020 on sn-devel-184