s3-libads Default to NOT using the server-supplied principal from SPNEGO

s3-libads Default to NOT using the server-supplied principal from SPNEGO

This principal is not supplied by later versions of windows, and using
it opens up some oportunities for man in the middle attacks.  (Becuase
it isn't the name being contacted that is verified with the KDC).

This adds the option 'client use spnego principal' to the smb.conf (as
used in Samba4) to control this behaivour.  As in Samba4, this
defaults to false.

Against 2008 servers, this will not change behaviour.  Against earlier
servers, it may cause a downgrade to NTLMSSP more often, in
environments where server names are not registered with the KDC as
servicePrincipalName values.

Andrew Bartlett
(cherry picked from commit bb7806283e71f3b8029aae0eed326b5847a36d83)