Added a hook for wakeup/user input.
I wanted to add a hook that got called on all user-input, but any
non-trivial use of that hook incurred too much overhead (about 7 ms per
paragraph for entry into perl, and another 100 ms or so per paragraph
for execution of the hook; tests run on pasting lorem ipusm, on
linerva).
This hook is called at most once a second (approximately).
