src/msg/async/EventKqueue.{h,cc} Added code to restore events on (thread)fork

src/msg/async/EventKqueue.{h,cc} Added code to restore events on (thread)fork

According the FreeBSD man page of kqueue(), the kq-descriptors become invalid
upon fork. It looks like the same happens when a kq-handle is created and then
a thread is created.

So we keep a list of assigned events with each kq-handle and when the handle
has beccome invalid, we recreated the kq-handle and the events that go with it.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>