input.conf: make `u` toggle between force and yes

There was a discrepancy in what the keybind was advertised to do in the
manual, and what the comment in input.conf described it to be doing. It
makes very little sense to add a keybind that changes the default and
doesn't allow you to get back to the default. This keybind is much more
useful if it toggles between yes/force instead of no/force.