Reduce code duplication in extrinsics-protect-items code

The same checks were being repeated for every damage type; this
sends them through two centralised functions (one for checking
whether an extrinsic blocks a specific instance of item destruction
and one for the enlightenment message), so that new mechanisms of
item destruction prevention will need to change only one point in
the code.

src/dothrow.c

@@ -2524,8 +2524,9 @@ breakobj(
 }
 
 /*
- * Check to see if obj is going to break, but don't actually break it.
- * Return 0 if the object isn't going to break, 1 if it is.
+ * Check to see if obj (which has just hit hard something at speed, e.g.
+ * thrown or dropped from height) is going to break, but don't actually
+ * break it. Return 0 if the object isn't going to break, 1 if it is.
  */
 boolean
 breaktest(struct obj *obj)