genmidi: Improve music for GOTHICDM MAP06.

This music sounded really horrible with Freedoom's GENMIDI lump, which
was a shame because this is a really beautiful level. So adjust a
couple of the more prominent instruments used for this music track:

 * Instrument 104 (Sci-fi):
   This instrument was using a frequency multiplier that put it 2
   octaves and a 5th above, making it (intentionally, because it's
   a "sci-fi" effect?) discordant. Rework this so that it isn't
   discordant any more.
 * Instrument 51 (SynthStrings #1):
   This had a slow attack phase which had the effect of offsetting
   the notes slightly, so that they seemed out of sync. Adjust this
   and also increase the feedback so that the instrument sounds more
   "stringy" (?)