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.

cmd-kill-server.c

@@ -43,7 +43,7 @@ const struct cmd_entry cmd_kill_server_entry = {
 int
 cmd_kill_server_exec(unused struct cmd *self, unused struct cmd_ctx *ctx)
 {
-	sigterm = 1;
+	kill(getpid(), SIGTERM);
 
 	return (0);
 }