Revert cond_signal() fix

It seems it yields to missing wake-up events with the
result of SF loosing on time as reported by many people.

So revert the patch and use a more robust approach: assume
there can be spurious wake ups events and make the code to
work also in those cases.

While debugging I found that WaitForSingleObject() had wrong
parameter 0 instead of INFINITE yielding to a crash while
exiting under Windows, strangely unnoticed til now.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>