update berkeley-db to 5.1

* modules/berkeley-db/bdb.c (add_message): fix
(DB-VERSION): use db_full_version when available
(DBE-CREATE): remote is no longer supported by upstream
(dbt_to_object): also return nil when p_dbt->size==0
(DB-COMPACT): add
(DB-VERIFY): mark the db handle invalid because it "may not be accessed
again after DB->verify() is called, regardless of its return"