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;
     }
 }
 

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;
     }
 }
 

src/priest.c

@@ -20,6 +20,7 @@ newepri(struct monst *mtmp)
     if (!EPRI(mtmp)) {
         EPRI(mtmp) = (struct epri *) alloc(sizeof(struct epri));
         (void) memset((genericptr_t) EPRI(mtmp), 0, sizeof(struct epri));
+        EPRI(mtmp)->parentmid = mtmp->m_id;
     }
 }
 

src/shknam.c

@@ -561,6 +561,7 @@ neweshk(struct monst *mtmp)
     if (!ESHK(mtmp))
         ESHK(mtmp) = (struct eshk *) alloc(sizeof(struct eshk));
     (void) memset((genericptr_t) ESHK(mtmp), 0, sizeof(struct eshk));
+    ESHK(mtmp)->parentmid = mtmp->m_id;
     ESHK(mtmp)->bill_p = (struct bill_x *) 0;
 }
 

src/vault.c

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