CVE-2016-2110: auth/ntlmssp: don't allow a downgrade from NTLMv2 to LM_AUTH

man smb.conf says "client ntlmv2 auth = yes" the default disables,
"client lanman auth = yes":

  ...
  Likewise, if the client ntlmv2 auth parameter is enabled, then only NTLMv2
  logins will be attempted.
  ...

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11644

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
