Before this change, the detach-keys were not validated, and the code either
fell back to the default sequence, or returned an obscure error if the
invalid sequence would produce an error on the daemon;

Before this patch:

    docker run -it --rm --detach-keys=shift-a,b busybox
    unable to upgrade to tcp, received 400

With this patch:

    docker run -it --rm --detach-keys=shift-a,b busybox
    invalid detach keys (shift-a,b): Unknown character: 'shift-a'

Note that the "unable to upgrade to tcp, received 400" error is still
something to be looked into; the client currently discards error messages
coming from the daemon.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>