Merge branch 'et/http-proxyauth' into maint

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