http: allow selection of proxy authentication method
CURLAUTH_ANY does not work with proxies which answer unauthenticated requests
with a 307 redirect to an error page instead of a 407 listing supported
authentication methods. Therefore, allow the authentication method to be set
using the environment variable GIT_HTTP_PROXY_AUTHMETHOD or configuration
variables http.proxyAuthmethod and remote.<name>.proxyAuthmethod (in analogy
to http.proxy and remote.<name>.proxy).