Make monster destroy armor -spell erode armor first

Instead of outright destroying the armor, the spell will instead
first erode the armor.  The spell hits 2-4 times, so if it hits
the same armor 4 times, it will get destroyed.  This does not
hit erodeproof armor.

Also change the scroll of destroy armor, so that blessed one will
destroy a cursed armor, if hero is only wearing that.

include/obj.h

@@ -451,6 +451,7 @@ struct obj {
 #define BURIED_TOO 0x2
 
 /* object erosion types */
+#define ERODE_NONE -1
 #define ERODE_BURN 0
 #define ERODE_RUST 1
 #define ERODE_ROT 2