Handle a disconnecting tincd better.

Handle a disconnecting tincd better.

- Try to prevent SIGPIPE from being sent for errors sending to the control
  socket. We don't outright block the SIGPIPE signal because we still want the
  tinc CLI to exit when its output is actually sent to a real (broken) pipe.

- Don't call exit() from top(), and properly detect when the control socket is
  closed by the tincd.