Reimplement LDBM_SYNC/LDBM_NOSYNC code (dbnosync).

Reimplement LDBM_SYNC/LDBM_NOSYNC code (dbnosync).
Old code applied sync flags to store(), however supported DBMs
require such flags to be specified during open().  The new
code now applies flags in ldbm_cache_open (which calls ldbm_open).
ldbm_cache_close() now calls ldbm_sync().  This will force
a updating of on-disk contents after each LDAP operation.
The old code either failed to sync the on-disk contents until
close or synced on every store.   Per LDBM operation syncing
*should* be safe enough... real data safety requires transactions.
Removed nosync option from BDB2 as it is not compatible with
txn support.
Also added code to disable DBM level locking as slapd is only
process acessing the databases (dbnolocking).