Upgrade to a modern HTTP client with portable HTTPS support

[emk]

Right now, we rely on OpenSSL to handle https, which means that we need to special-case everything for the Mac. Ideally, this would mean using native-tls, which would in turn involve replacing our ancient hyper with reqwest.

Unfortunately...

We need to handle non-TCP Docker sockets, which means we're providing a custom network transport. And reqwest doesn't support this. At the moment, we use:

• unix_socket for Unix-like systems. This is synchronous only, but it might be possible to switch to hyperlocal.
• named_pipe for Windows. This might be replaced with something based on mio-named-pipes.

So there's probably a moderate amount of hyper-related code to get us talking securely to Docker again.