Add support for SSL-enabled apt servers

This is to allow for non-apt-cacher caching servers (i.e. squid) that may be put in place as a simple apt proxy.  Since squid properly supports HTTPS proxying via the CONNECT method, we can continue to proxy our apt requests through our proxy server.

As to why you would want to do this: if you have a proxy set up as an ACL filter, this allows you to use it for all of apt, rather than just plaintext sources.