force modifier keys off?

[Tom Horsley]

My system just became convinced that the control and shift
modifier keys were pressed all the time (don't ask me how
it got in this state). I finally just rebooted to set
everything back to normal.

Is there any handy utility I can use to tell the X server:
"Trust me, there are no modifier keys pressed."? If it
happens again, it would be nice to have it so I don't
have to reboot to fix it :-).

Maybe use the X test extension to send key release events
for all modifier keys?