Bug 12974: Disable NTLM and Negotiate HTTP Auth

This is technically an embargoed Mozilla bug, so I probably shouldn't provide
too many details.

Suffice to say that NTLM and Negotiate auth are bad for Tor users, and I doubt
very many (or any of them) actually need it.

The Mozilla bug is https://bugzilla.mozilla.org/show_bug.cgi?id=1046421