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.

Docker-DCO-1.1-Signed-off-by: Ben Firshman <ben@firshman.co.uk> (github: bfirsh)