http: allow gssapi-negotiate authentication

If either the config option http.gssNegotiate or the environment variable
GIT_CURL_GSSNEGOTIATE are set, enable gssapi-negotiate authentication in libCURL
with the CURLAUTH_GSSNEGOTIATE option. This is similar to calling curl with the
--negotiate flag and a fake user. This allows git to use http authentication
with mechanisms that support gssapi-negotiate authentication like Kerberos5.