Merge branch 'et/http-proxyauth' into maint

We used to ask libCURL to use the most secure authentication method available when talking to an HTTP proxy only when we were told to talk to one via configuration variables. We now ask libCURL to always use the most secure authentication method, because the user can tell libCURL to use an HTTP proxy via an environment variable without using configuration variables. * et/http-proxyauth: http: always use any proxy auth method available