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 #().
Rather than running status-left, status-right and window title #() with popen
immediately every redraw, queue them up and run them in the background,
starting each once every status-interval. The actual status line uses the
output from the last run.
This brings several advantages:
- tmux itself may be called from inside #() without causing the server to hang;
- likewise, sleep or similar doesn't cause the server to block;
- commands aren't run excessively often when redrawing;
- commands shared by status-left and status-right, or used multiple times, will
only be run once.
run-shell and if-shell still use system()/popen() but will be changed over to
use this too later.
