Initial changes to move tmux to libevent.
This moves the client-side loops are pretty much fully over to event-based only
(tmux.c and client.c) but server-side (server.c and friends) treats libevent as
a sort of clever poll, waking up after every event to run various things.
Moving the server stuff over to bufferevents and timers and so on will come
later.
Add a pipe-pane command to allow a pane to be piped to a shell command, for
example:
pipe-pane 'cat >~/out'
No arguments stops outputing and closes the pipe; the -o flag toggles a pipe
and on and off (useful for key bindings).
Suggested by espie@.
