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 flag for jobs that shouldn't be freed after they've died and use it for
status jobs, then only kill those jobs when status-left, status-right or
set-titles-string is changed.
Fixes problems with changing options from inside #().
Split the server code handling clients, jobs and windows off into separate
files from server.c (merging server-msg.c into the client file) and rather than
iterating over each set after poll(), allow a callback to be specified when the
fd is added and just walk once over the returned pollfds calling each callback
where needed.
More to come, getting this in so it is tested.
