Enables Edge-Triggered behaviour for the polling methods
which support it (currently only epoll and kqueue).
The user event handler is notified of Edge-Triggered behavior by specifying EV_ET flag in event type.
The user should be aware of possible consequences of
using Edge-Triggered behaviour (see epoll and kqueue manual), i.e IO starvation. However if used properly could provide additional performance gain.
Tested on FreeBSD 7.0, Ubuntu 7.04 with Linux kernel 2.6.15.