Import from SAMBA's tdb:

Import from SAMBA's tdb:
commit b90863c0b7b860b006ac49c9396711ff351f777f
Author: Howard Chu <hyc@highlandsun.com>
Date:   Tue Mar 31 13:15:54 2009 +1100

    Add tdb_transaction_prepare_commit()

    Using tdb_transaction_prepare_commit() gives us 2-phase commits. This
    allows us to safely commit across multiple tdb databases at once, with
    reasonable transaction semantics

    Signed-off-by: tridge@samba.org