dmxgus: Fix numbering of percussion instruments.

The indices of the percussion instruments were all off-by-one. These
should be 128+genmidi number, but were 127+ by mistake. This
considerably improves the percussion in Freedoom when using GUS.
dmxgus: Improve percussion groupings.

Regroup GUS percussion instruments based on personally having listened
to the samples in the GUS instrument set and identifying similar-
sounding instruments.
dmxgus: Add 'do not edit' message.

As the DMXGUS lump is autogenerated, add an explicit instruction not
to edit the lump, as someone might do so, naively seeking to improve
it.