r15426: Implement SPNEGO as the default RPC authentication mechanism. Where

r15426: Implement SPNEGO as the default RPC authentication mechanism.  Where
this isn't supported, fallback to NTLM.

Also, where we get a failure as 'logon failure', try and do a '3
tries' for the password, like we already do for CIFS.  (Incomplete:
needs a mapping between RPC errors and the logon failure NTSTATUS).

Because we don't yet support Kerberos sign/seal to win2k3 SP1 for
DCE/RPC, disable this (causing SPNEGO to negotiate NTLM) when kerberos
isn't demanded.

Andrew Bartlett