sspi: Only call CompleteAuthToken() when complete is needed

Don't call CompleteAuthToken() after InitializeSecurityContext() has
returned SEC_I_CONTINUE_NEEDED as this return code only indicates the
function should be called again after receiving a response back from
the server.

This only affected the Digest and NTLM authentication code.
sspi: Renamed expiry time stamp variables

Code cleanup to try and synchronise code between the different SSPI
based authentication mechanisms.
sspi: Renamed max token length variables

Code cleanup to try and synchronise code between the different SSPI
based authentication mechanisms.
sspi: Synchronization of cleanup code between auth mechanisms