Currently we always map any incoming domain to our own domain
in map_user_info_cracknames(), so that the winbind module is never
used at all, e.g. we're DC of W4EDOM-L4.BASE with a forest trust to W2012R2-L4.BASE:
    
      [2017/03/22 10:09:54.268472,  3, pid=4724, effective(0, 0), real(0, 0)] ../source4/auth/ntlm/auth.c:271(auth_check_password_send)
        auth_check_password_send: Checking password for unmapped user [W2012R2-L4]\[administrator]@[UB1404-163]
      [2017/03/22 10:09:54.268496,  5, pid=4724, effective(0, 0), real(0, 0)] ../source4/auth/ntlm/auth_util.c:57(map_user_info_cracknames)
        map_user_info_cracknames: Mapping user [W2012R2-L4]\[administrator] from workstation [UB1404-163]
        auth_check_password_send: mapped user is: [W4EDOM-L4]\[administrator]@[UB1404-163]