s3:winbind: Refactor append_unix_username(), do not take winbindd_response struct...

s3:winbind: Refactor append_unix_username(), do not take winbindd_response struct as parameter

Refactor the append_unix_username() function to do not take a
winbindd_response struct as parameter but its members. The
unix username is returned as an out parameter and the caller is
responsible for setting it in the winbindd_response struct.

Later winbindd_dual_pam_auth() will be converted to a local RPC
call handler and the netr_Validation will be returned in the 'r' struct
from the child to the parent. The parent will then fill the
winbindd_response struct.

Signed-off-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>