Multi Port MIDI Support

[spessasus]

Related discussion

I've started this discussion: #1360

Since #1352 was closed and not really a feature request, this is a sort of "official" feature request for this.

Describe the solution you'd like

Fluidsynth should recognize the 0x21 meta message.

Describe alternatives you've considered

N/A

Additional context

https://github.com/spessasus/SpessaSynth/wiki/About-Multi-Port