uselib is no longer implicit (inherited by .use). This is still incomplete,
some uselibs for non-linux variants may be missing.
bld.is_defined("HAVE_XXX") also no longer works and will have to be
changed (I think to bld.env["HAVE_XXX"]) in countless places.
* allow to change buffersizes
* subscribe to buffersize & samplerate changes
* add support for half-duplex devices.
* aggregate Devices (not yet used) code from JACK2
* unify deprecated API wrappers
* properly keep track of MIDI ports
* disable MidiI/O during freewheeling
* various small fixes & cleanup
