Merge fire-based erosion to common codepaths.

This finally eliminates all direct increases of `oeroded` and `oeroded2`
and moves them all to go via `erode_obj()`. They are still manipulated
directly in a few places, but not to erode objects.

This now merges the `fire_damage()` function to a common codepath, used
for items on lava and burning oil, but fire needs more work. There is
still a duplication between `destroy_item()` and `fire_damage()`; the
two codepaths should eventually be merged in some manner so that there
is only one codepath to say "an object was affected by fire". This path
might require some parameters, such as whether the fire will just erode
objects or burn them outright, but that can happen another day.

include/obj.h

@@ -325,6 +325,19 @@ struct obj {
 #define ERODE_ROT 2
 #define ERODE_CORRODE 3
 
+/* erosion flags for erode_obj() */
+#define EF_NONE    0
+#define EF_GREASE  0x1 /* check for a greased object */
+#define EF_DESTROY 0x2 /* potentially destroy the object */
+#define EF_VERBOSE 0x4 /* print extra messages */
+#define EF_PAY     0x8 /* it's the player's fault */
+
+/* erosion return values for erode_obj(), water_damage() */
+#define ER_NOTHING   0 /* nothing happened */
+#define ER_GREASED   1 /* protected by grease */
+#define ER_DAMAGED   2 /* object was damaged in some way */
+#define ER_DESTROYED 3 /* object was destroyed */
+
 /*
  *  Notes for adding new oextra structures:
  *