Allow ctrl-p ctrl-q customization

Closes #3519

@jamtur01 @SvenDowideit: documentation is coming, I want to make sure everyone agrees on the feature first.

This PR allows you to change the set of keys you have to type to detach in TTY mode, it can be less or more that two.

The basic ASCII table is supported, for example you can do: `docker -d --detach-keys="ctrl-@"` and detach in one keystroke or `docker -d --detach-keys="DEL,+"` if you don't like to have a  `ctrl` key :smile: 
Actually, maybe we don't need the `else` part since it is redundant with the default value of the `flDetachKeys` flag in docker.go. By default, detachKeys will be non-empty and thus we would never enter the `else` codepath.