libfetch: support proxies for https

[Jonathan Perkin <jperkin%joyent.com@localhost>]

Another issue that cropped up after enabling https for our 2016Q3 is
that proxies are no longer honoured.

I've merged back support for these from upstream libfetch here:

  https://github.com/joyent/pkgsrc/commit/37e99539

Please review and I'll try to get both this and the read issue merged
soon.

Thanks,

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com