Listen for MIDI event through ALSA "virtual" rawmidi interface

This allows for connecting up any MIDI device (hardware or software)
through programs such as aconnect or qjackctl that allow one to
connect MIDI outputs to inputs. This is quite convenient for
connecting a virtual keyboard (such as vkeybd or vmpk) to scherzo.

It can also be used to connect a real, hardware MIDI keyboard to
scherzo, but we keep around the code that does a direct connection to
/dev/midi1 since that simplifies things by reducing one step in the
usage, (the user needn't run aconnect nor qjackctl to make the
connection).