add mexta field to simplify detection of overwrite

Because this invalidates existing save and bones files,
an increment of EDITLEVEL will accompany this.

src/dog.c

@@ -27,6 +27,7 @@ newedog(struct monst *mtmp)
     if (!EDOG(mtmp)) {
         EDOG(mtmp) = (struct edog *) alloc(sizeof(struct edog));
         (void) memset((genericptr_t) EDOG(mtmp), 0, sizeof(struct edog));
+        EDOG(mtmp)->parentmid = mtmp->m_id;
     }
 }