epoll: add EPOLLEXCLUSIVE constant and documentation

epoll: add EPOLLEXCLUSIVE constant and documentation

I'm still not convinced this is necessary, but it's in Linux, now,
and maybe some folks will want it.

My position remains:  Never put listen sockets in epoll if
you're using threads; instead, dedicate a thread to doing
nothing but _blocking_ accept4 + epoll_ctl(.. EPOLL_CTL_ADD).

Of course, if you're using something which does not have
native thread support but using multiple processes, then
yes, EPOLLEXCLUSIVE will prevent multiple processes from
turning into a thundering herd.

In other words: I would use this with Perl5, but not with Ruby 1.9+ :P