NTLM helpers cleanup pt 3: migrate libsmbval into libntlmauth

NTLM helpers cleanup pt 3: migrate libsmbval into libntlmauth

Library changes:
* ntlmauth.* files moved to libntlmauth/

* helpers/ntlm_auth/smb_lm/smbval moved to libntlmauth/

* No behaviour changes. Since I can't test the deeper logics.
  Just enough to make the code built with portable types available in Squid

* API shuffled slightly to use less .h and to remove all external uses of
  private *-priv.h definitions.

Library now provides three NTLM backend API:
  libntlmauth/ntlmauth.h  - NTLM packet handling
  libntlmauth/smb.h       - SMB LM credential validation
  libntlmauth/rfcnb.h     - RFCNB (NetBIOS) domain server communications

Helper Changes:

* NTLM helpers tweaked slightly to build with the adjusted libntlmauth API
  and ntlm_smb_lm_auth helper to build as C++

* automake logics updated to obey --disable-auth and --disable-auth-ntlm

NOTE: There will be extra code safety and testing benefits gained by
      converting libntlmauth to C++ as well. But that requries someone who
      can test the code behaviour during the upgrade. For now this wil do.