krb5: Do not clobber keytab entry timestamps

We set the timestamp field of krb5_keytab_entry in every case in-tree,
so we should not clobber it in krb5_kt_add_entry().  This is very
important in the context of virtual service principals, as the timestamp
of the keys in the keytab is a clue to when they must be refetched!

lib/krb5/keytab.c

@@ -883,7 +883,8 @@ krb5_kt_add_entry(krb5_context context,
 			       id->prefix);
 	return KRB5_KT_NOWRITE;
     }
-    entry->timestamp = time(NULL);
+    if (entry->timestamp == 0)
+        entry->timestamp = time(NULL);
     return (*id->add)(context, id,entry);
 }