Meta event 0x21, port prefix, treated as unknown meta event

[arfy8820]

Hello.
I have midi files that use meta event 0X21, port prefix, to select among multiple midi devices on a midi bus. This was a common way to get around the 16-channel limit in 90s/early 2000s era midi sequencers. However, currently, libmidi treats these events as unknown meta events.
The way to handle these is the same as the existing channel prefix, 0X20 events. Range is 0-255.