Re-evaluate TLS (and possibly HTTP) settings and defaults used in the codebase

[mtrmac]

Outstanding comments from #160:

• Why are we using a ServerDefault tls.Config in a client?
• Why are we applying it only to a single path, not when we override tls.Config for our purposes?
• In tlsclientconfig.NewTransport, the proxyDialer setup should happen before we create a http.Transport, so that we don’t need to override tr.Dial in a created object. Irrelevant after https://github.com/containers/image/pull/1748