This made the shutdown procedure a little more interesting since
BufferedReader's read(Line) cannot easily be interrupted. One possible
solution is to use shutdownInput() on the socket to provoke an End of
Stream [1], which is what is done here. The other way is to just close
the socket, causing an IOException. I opted not to do that since I would
have to suppress all IOExceptions in basically the entire run() method
in LCDSocketPoller

[1]: https://stackoverflow.com/questions/3595926/how-to-interrupt-bufferedreaders-readline

Access to _lastLine is now properly synchronized. It is a bit ugly to
use a field just to send back a status line once, but it seems the
easiest way in the current implementation. It is also not customary to
increment a semaphore more times than you plan on decrementing it, but
it's not strictly illegal either.

LCD::connect now only tries to connect once. Connection issues should be
rare, and it is up to the application what to do if the first attempt
should fail.