spnego: Support proxy authentication

[andybalholm]

I'd like to add support for HTTP proxy authentication. I think the changes would basically be limited to header names and status codes:

Authorization -> Proxy-Authorization
WWW-Authenticate -> Proxy-Authenticate
http.StatusUnauthorized -> http.StatusProxyAuthRequired

The biggest question is where to put the switch between the two modes. I would suggest a new function, perhaps SPNEGOKRB5ProxyAuthenticate. Then that function and SPNEGOKRB5Authenticate would both call an unexported function with an extra parameter which would switch between regular HTTP authentication and proxy authentication.

What do you think?

[nrolans]

I needed similar flexibility for other features and suggested PR #391. I think your use case could technically be integrated in that approach too. However be aware that it is currently pending review.

[sandy2008]

Hi! Please take a look at this PR: #535

[andybalholm]

I'm glad to see it's being added, but we already found another solution to our problem, so I can't really test it at this point.