Fixes for Knight keyboard.
The timing is delicate: shift-register does not tolerate the delay that
the Space Cadet requires and does apparently include the initial low
wakeup.
Since there are not key-up signals, a doubled key-down requires an
separate empty report to the host. README needs to remind user to
disable auto-repeat.
Add more keysyms for the graphic characters. Do not attempt to do this
for the slightly-odd numeric shifts. Maybe address those in operating
system's layout?