spnego_sspi: add support for channel binding

Attempt to add support for Secure Channel binding when negotiate
authentication is used. The problem to solve is that by default IIS
accepts channel binding and curl doesn't utilise them. The result was a
401 response. Scope affects only the Schannel(winssl)-SSPI combination.

Fixes https://github.com/curl/curl/issues/3503
Closes https://github.com/curl/curl/pull/3509