(mu_hdrent_find): Allow negative instance indices, meaning scanning from the bottom up.

@@ -75,9 +75,21 @@ static struct mu_hdrent *
 mu_hdrent_find (struct _mu_header *hdr, const char *name, int pos)
 {
   struct mu_hdrent *p;
+
+  if (pos > 0)
+    {
       for (p = hdr->head; p; p = p->next)
 	if (strcasecmp (MU_HDRENT_NAME (hdr,p), name) == 0 && pos-- == 1)
 	  break;
+    }
+  else if (pos < 0)
+    {
+      for (p = hdr->tail; p; p = p->prev)
+	if (strcasecmp (MU_HDRENT_NAME (hdr,p), name) == 0 && ++pos == 0)
+	  break;
+    }
+  else
+    p = NULL;
   return p;
 }