This patch adds selectable EdgeTriggered/LevelTriggered behaviour to
epoll.

It has been widely discussed on lkml about two weeks ago and everyone
very welcome the change.  It has been even more widely discussed through
private emails with application developers, that do not feel confortable
posting on lkml.  The great value of the patch is that selecting the LT
behaviour, applications using poll/select can be ported very easily to
epoll, making existing apps to benefit from epoll scalability with very
short ETA's.

The API remains the same with the addition of a EPOLLET event flag that
sets the LT/ET behaviour for that fd.