ALSA code cleanup/standardisation.

@terminx, applied your patch for testing into a separate branch and added a commit to make it all compile. For posterity, the build errors I received are here.

The biggest issue I encountered was with returning a pointer in the function ALSADrv_MIDI_ListPorts(). I also needed to const the char string in the alsa_mididevinfo_t struct, which made the macro DO_FREE_AND_NULL() not work.

The two commits here produce compiled and functioning code.