libevent: split event_list

libevent: split event_list

Rather than keeping poll events on the same list as events handled by
libevent, split the list in two.
This means we don't need POLL_FD to identify the poll events.
That, combined with the use of event_get_fd() means we don't need to
store the fd in the event struct.  Just the 'num' is sufficient.

Signed-off-by: NeilBrown <neil@brown.name>