Deprecate support for --dockerport flag.

As the result of long standing issues with support for half-close of inbound
TCP connections to Virtual Box guests on (at least) OSX use of port-forwarded
docker connections should be discouraged.

So, if the user really wants to configure it, let them. Otherwise, don't
set it up in the first place.

See boot2docker issue #150 (and others) for details.

Docker-DCO-1.1-Signed-off-by: Jon Seymour <jon.seymour@gmail.com> (github: jonseymour)