music fixes for PC-DOS original Lemmings

This fixes the problem that in the original game, the music always revert to Fun 1's music after restarting the current level.  There are 3 ways in which this problem can be addressed:

FIX #1:

This is similar to the approach found in PC-DOS ONML, and the CD version of DOS Lemmings.  Music is not tied to the level.  Instead, the game simply go to the next music every time you win a level, and otherwise stay at the same music.  So for example, if you start off with Fun 1, win that level, and then use passwords to go back to Fun 1 again, the music will still be changed.

In the CD version of DOS Lemmings, it's also the case that the game does not go to the next music when the next level in question is one of the four special graphics levels (which have their own specific music).  This fix follows that behavior also.



FIX #2:

This is similar to the approach found in the Amiga, Mac, SNES, etc. versions of Lemmings.  Each level is mapped to a music, so that you always hear the same music when playing the same level no matter how you reach that level.  The music is assigned so that Fun 1-17 gets the 17 music, and then Fun 18 repeats Fun 1, etc.  The special graphics level have their own music in place of the one that's normally assigned, without affecting how the surrounding level's music gets assigned.  In other words, Fun 21 gets the same music as Fun 4; Fun 22 should get Fun 5's music but being a special graphics level, it uses its own music instead; Fun 23 gets the same music as Fun 6.



FIX #3:

This is a minor variation of FIX #2, where music are tied to levels.  The only difference is how the music assignment interacts with special graphics levels.  In this fix, what would happen for example is that Fun 23 gets the same music as Fun 5, rather than Fun 6.