Factor out common code in the NTLMSSP/SPNEGO code.

Factor out common code in the NTLMSSP/SPNEGO code.

The idea here is to seperate, as much as possible, the SPNEGO layer from the
NTLMSSP layer.  This not only helps us with protocol correctness, but also
should allow further mechinisms to be added with relitive ease.  I indend to
make the kerberos code use this shortly.

I've never seen the 'zero length blob' form of the anonymous login, so I've
removed that case.

Andrew Bartlett
(This used to be commit a8773c9f825539c5bc17e4200b16d7ebbe0b7620)