Split monster goal coordinate out of mstrategy field

Instead of packing a coordinate into unsigned long, store the goal in
a coord struct, making the code a bit cleaner.  Monster struct is
of course slightly bigger, but that should not really matter.
No change in monster behaviour.

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 119
+#define EDITLEVEL 120
 
 /*
  * Development status possibilities.