Don't lock the screen on a double scroll lock.

A double scroll lock is used to communicate with my KVM.  This should
not have a side effect of locking the screen, so arrange for the screen
to only lock if the scroll lock key is pressed once.

Unfortunately, FVWM does not appear to have "single press" versus
"double press" for key bindings (although it does for mouse
bindings...), so we simulate it using a timer.