Refactor OFKernelEventObserver

This moves the locked queue for actions to
OFKernelEventObserver_LockedQueue, which is now used for select and
poll, but skipped for kqueue and epoll, as they natively support changes
from another thread.

This fixes the problem that removing an object is delayed until the next
observe call - at which point it might have already been closed, meaning
the fd is no longer available. This was mainly a problem with kqueue, as
closing the fd already removed it from the kqueue, which then resulted
in an error being returned when trying to manually remove the fd from
the kqueue.