music: Fix -dumpsubstconfig for Heretic/Hexen.

The config dumping command line option assumed that music lumps were
named like D_MYLUMP, but this is not the case for Heretic and Hexen,
where there is no D_ prefix and music lumps can have any name.

Change the logic to instead look at the contents of lumps and identify
music lumps from the MUS / MIDI header.