linux: list midiports to stdout + check for default port using MIDI_IN env-var

[coderofsalvation]

Linux midi-input was hardcoded to midiport 0 (which is a virtual thru-device on most *NIX systems).
This still requires other software to setup midi-routing (aconnectgui, carla etc).
This is no longer needed since this PR will:

• list all available midiports
• optionally directly connect to a particular port using MIDI_IN=x environment variable.

Ideally this would be done thru gui-popups but that's significantly more code to implement/maintain.

$ export MIDI_IN=1
$ ./milkytracker                                                                                                                                          
SDL: Using audio driver: pulseaudio                                                          
SDL: Buffer size = 128 samples (requested 128) 
MIDI: scanning input ports:                                                                  
MIDI:  0. Midi Through:Midi Through Port-0 14:0                                                                                                                                           
MIDI:  1. electribe2 sampler:electribe2 sampler electribe2 s 20:0                            
MIDI: selecting MIDI-in port: 1                                                                                                                                                           
MIDI: run `MIDI_IN=x ./milkytracker` to select different port)   

[coderofsalvation]

thx! merged in v1.04