Backport proxy support to the curl downloader.

This pull request backports proxy support from the master branch for our upcoming 2.1.1 release. Once this is merged to the 2.1 branch, I will merge it back to master with the "ours" strategy to avoid weirdness since something quite similar to this is already in master.

There will be another pull request in the pulp_rpm project that takes advantage of this feature.