closes SF #514433
can now pass 'None' as the filename for the bsddb.*open functions,
and you'll get an in-memory temporary store.
docs are ripped out of the bsddb dbopen man page. Fred may want to
clean them up.
Considering this for 2.2, but not 2.1.
BTree databases, the key need not be in the database. Also, tell about
the exception if the key is not in the DB for other DB types.
This closes SF bug #127377.