ALSADriver: Advertise as `MIDI_GENERIC` as well as MT32

Some programs, such as the virtual piano [VMPK](http://vmpk.sourceforge.net/)
show only midi outputs with `SND_SEQ_PORT_TYPE_MIDI_GENERIC`.

The [alsa documentation](http://www.alsa-project.org/alsa-doc/alsa-lib/group___seq_port.html)
for `SND_SEQ_PORT_TYPE_MIDI_GENERIC` says "This port understands MIDI
messages." It does not mean General Midi.

So this seems to imply that a MT32 synth has to advertise both as
`MIDI_GENERIC` and as `MIDI_MT32`.
Merge pull request #17 from laanwj/master

ALSADriver: Advertise as `MIDI_GENERIC` as well as MT32