Re-adding an event to a kqueue modifies the parameters of the original event.

Re-adding an event to a kqueue modifies the parameters of the original event.
However, if the original knote had been disabled then it is not automatically
re-enabled.

Fix this by using EV_ADD to create an mevent and EV_ENABLE to enable it.

Adding a kevent for the first time implicitly enables it so existing callers
of mevent_add() don't need to change.

Reviewed by:	grehan