From: Oliver Xymoron <oxymoron@waste.org>

This fixes several calculation errors and races in entropy accounting
that would allow /dev/random output to greatly exceed the measured
entropy collection. This doesn't include any of my more controversial
hardening, it just makes it behave as intended.

It also corrects the operation of the 'catastrophic reseeding' feature
so that it will actually prevent the state extension attack it's meant
to guard against.

And finally, it also fixes a couple missed wake-up and accidental
sleep bugs uncovered by the above fixes.

Debug instrumentation has been improved to help verify correctness.