it calls poll or select. This allows the front end to iterate over
the events instead of having that code in both back ends. It will
also simplify support for timeout events. Also make sure we can't
touch freed memory if a callback frees its own event.
queue functions. This includes a private queue.h header derived
from FreeBSD. It is simpler to just use our own header rather than
try to deal with macros that may or may not be present in various
queue.h incarnations.
simplied subset of libevent2. Currently only fd events are supported
(since that's all we need). The poll() backend is used by default,
except on Mac OS X where poll() is broken for devices (including
/dev/tty and ptys).
