Correct key bindings to Control-<foo>

Correct key bindings to Control-<foo>

Alberto Bertogli reported on #git that git-gui was exiting with
alt-q, while gitk on the same system was exiting with ctrl-q.
That was not what I wanted.  I really wanted M1B to be bound to
the Control key on most non-Mac OS X platforms, but according to
Sam Vilain M1 on most systems means alt.  Since gitk always does
control, I'm doing the same thing for all non-Mac OS X systems.

Signed-off-by: Shawn O. Pearce <redacted>