LDBM changes

["Kurt D. Zeilenga" <Kurt@OpenLDAP.org>]

After some discussions with Keith Bostic of Sleepycat, I
made a few changes to back-ldbm to address a threading
issue which could lead to corruption of the DB.  Basically,
we need to either use the ldbm_big_mutex, as we do with
GDBM and others, or use the Berkeley's Concurrent Database
features (DB_INIT_CDB).  As the latter was fairly easy to
enable and should perform better, that's what I did.

I'm going to leave it in HEAD for a few days to let others
abuse it before merging it into OPENLDAP_REL_ENG_2.  Any
testing you can do (before and/or after the merge) would
be appreciated.

Kurt