In 2007, we've added support for multiple lookup levels for LSA
LookupNames family of calls. However, forest-wide lookups, as described
in MS-LSAT 2.2.16, never worked because flags passed to lookup_name()
were always set to zero, expecting at least default lookup on a DC to
apply. lookup_name() was instead treating zero flags as 'skip all
checks'.

Allow at least own domain lookup in case domain name is the same.
This should allow FreeIPA DC to respond to LSA LookupNames3 calls from a
trusted AD DC side.

For the reference, below is a request Windows Server 2016 domain
controller sends to FreeIPA domain controller when attempting to look up
a user from a trusted forest root domain that attemps to login to the
domain controller. Notice the level in the lsa_LookupNames3 call and
resulting flags in lookup_name().

    [2019/08/03 07:14:24.156065,  1, pid=23639, effective(967001000, 967001000), real(967001000, 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                     : 0000004c-0000-0000-455d-3018575c0000
                  num_names                : 0x00000001 (1)
                  names: ARRAY(1)
                      names: struct lsa_String
                          length                   : 0x000a (10)
                          size                     : 0x000c (12)
                          string                   : *
                              string                   : 'XS\ab'
                  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)
    [2019/08/03 07:14:24.156189,  6, pid=23639, effective(967001000, 967001000), real(967001000, 0), class=rpc_srv] ../../source3/rpc_server/rpc_handles.c:339(find_policy_by_hnd_internal)
      Found policy hnd[0] [0000] 00 00 00 00 4C 00 00 00   00 00 00 00 45 5D 30 18   ....L... ....E]0.
      [0010] 57 5C 00 00                                        W\..
    [2019/08/03 07:14:24.156228,  4, pid=23639, effective(967001000, 967001000), real(967001000, 0)] ../../source3/smbd/sec_ctx.c:215(push_sec_ctx)
      push_sec_ctx(967001000, 967001000) : sec_ctx_stack_ndx = 2
    [2019/08/03 07:14:24.156246,  4, pid=23639, effective(967001000, 967001000), real(967001000, 0)] ../../source3/smbd/uid.c:552(push_conn_ctx)
      push_conn_ctx(0) : conn_ctx_stack_ndx = 0
    [2019/08/03 07:14:24.156259,  4, pid=23639, effective(967001000, 967001000), real(967001000, 0)] ../../source3/smbd/sec_ctx.c:319(set_sec_ctx_internal)
      setting sec ctx (0, 0) - sec_ctx_stack_ndx = 2
    [2019/08/03 07:14:24.156273,  5, pid=23639, effective(967001000, 967001000), real(967001000, 0)] ../../libcli/security/security_token.c:53(security_token_debug)
      Security token: (NULL)
    [2019/08/03 07:14:24.156285,  5, pid=23639, effective(967001000, 967001000), real(967001000, 0)] ../../source3/auth/token_util.c:865(debug_unix_user_token)
      UNIX token of user 0
      Primary group is 0 and contains 0 supplementary groups
    [2019/08/03 07:14:24.156311,  5, pid=23639, effective(0, 0), real(0, 0), class=rpc_srv] ../../source3/rpc_server/lsa/srv_lsa_nt.c:244(lookup_lsa_sids)
      lookup_lsa_sids: looking up name XS\ab
    [2019/08/03 07:14:24.156327, 10, pid=23639, effective(0, 0), real(0, 0)] ../../source3/passdb/lookup_sid.c:112(lookup_name)
      lookup_name: XS\ab => domain=[XS], name=[ab]
    [2019/08/03 07:14:24.156340, 10, pid=23639, effective(0, 0), real(0, 0)] ../../source3/passdb/lookup_sid.c:114(lookup_name)
      lookup_name: flags = 0x00