Extend the 'netlogon' CLDAP and NBT implementation.

Extend the 'netlogon' CLDAP and NBT implementation.

This now handles checking if the user exists, including validating the
ACB mask on the user.

This would be a nasty security hole, if Kerberos did not already
expose this information anonymously...

Andrew Bartlett
(This used to be commit 441b286c00f9a7743cdefeb243545bdbd2c94c5e)