Desirable features
------------------
- A way of temporarily closing an input or an output port. It would be useful
  to allow other applications access to hardware ports without having to shut
  down jackd.
- atomic data write to port for hardware i/o client
- Handle other poll event types?
- Split generic port buffer writing code out into separate private api
- create a libjackmidi.so that jack midi clients must link against.
- create a libjackbufferedport.so that libjackmidi.so must link against.
- separate out the port type registering stuff