Add support for tcps:// URLs

This makes it possible to enable TLS support on the client without having to pass any command line options -- you can use `DOCKER_HOST` with a `tcps://` URL.

On the client, this will behave as if the `--tlsverify` flag has been passed. On the server, the `--tls` flag. The difference between `--tls` and `--tlsverify` on the client seem a bit odd, and I *think* this makes sense for what you'd normally want to do.

In terms of tests, there are no unit tests that cover any of this code, and it seems wasteful to add an integration test to just check a single command-line option. My Go-fu is the limiting factor here, so any advice on how to write a good unit test would be appreciated.

This is an implementation of the proposal in #7415.