add mexta field to simplify detection of overwrite

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

src/minion.c

@@ -21,6 +21,7 @@ newemin(struct monst *mtmp)
     if (!EMIN(mtmp)) {
         EMIN(mtmp) = (struct emin *) alloc(sizeof(struct emin));
         (void) memset((genericptr_t) EMIN(mtmp), 0, sizeof(struct emin));
+        EMIN(mtmp)->parentmid = mtmp->m_id;
     }
 }