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-pipe-pane.c

@@ -88,7 +88,7 @@ cmd_pipe_pane_exec(struct cmd *self, struct cmd_ctx *ctx)
 	case 0:
 		/* Child process. */
 		close(pipe_fd[0]);
-		sigreset();
+		server_signal_clear();
 
 		if (dup2(pipe_fd[1], STDIN_FILENO) == -1)
 			_exit(1);