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: 
I did that for compatibility with old version, but maybe you're right, we could remove it since we don't support new client with old server.