Save and restore hero tracks

The tracks left by hero were cleared when player saved and
restored the game, or changed levels.  Now the tracks are
saved in the dungeon level, so changing levels keeps the tracks
left by hero in that level.

Also increased the length of tracks from 50 to 100, and
simplify the tracking function.

Thing not done: fade out old tracks when returning to a level.

Breaks saves and bones.

include/patchlevel.h

@@ -17,7 +17,7 @@
  * Incrementing EDITLEVEL can be used to force invalidation of old bones
  * and save files.
  */
-#define EDITLEVEL 90
+#define EDITLEVEL 91
 
 /*
  * Development status possibilities.