Make Tor use Niels Provos's libevent instead of it's current

poll-but-sometimes-select mess.  This will let us use faster async cores
(like epoll, kpoll, and /dev/poll), and hopefully work better on Windows
too.

There are some fairly nasty changes to main.c here; this will almost
certainly break something.  But hey, that's what alphas are for.


svn:r3341