Attempt a clearer design of modes

* darkroom.el (darkroom--turn-on): New function.
(darkroom--tentative-mode-driving): Removed this.
(darkroom-mode): Use new `darkroom--turn-on' and
`darkroom--turn-off'.
(darkroom-tentative-mode): Assert that no `darkroom-mode' and
`darkroom-tentative-mode' are mixed.
(darkroom--turn-off): New function.