Improve handling of NSURL authentication methods. Make "HTMLForm"
authentication a no-op as it should be instead of making it fall back to
Basic authentication.
Add stubs for NTLM and Negotiate IIS methods. They should be easily
implementable given the current framework for authentication methods.
Add stubs for ClientCertificate and ServerTrust authentication methods.
A blocker for supporting them is the lack of a Security.framework
implementation (since they rely on SecTrustRef). They would also require
additions to the current SSL handling code for GNUstep https requests.
Also stub the -distinguishedNames property of NSURLProtectionSpace,
which also deals with certificate-related authentication methods.
