ldb: pack_format_override option

ldb: pack_format_override option

For TDB databases, toggling GUID indexing mode will also toggle
pack format version 2. This provides a convenient downgrade path for
Samba databases, but the process doesn't work for MDB databases because
GUID indexing cannot be disabled when the MDB backend is used. This patch
addresses that corner case by providing support for a pack_format_override
option which will force the database to use pack format version 2.

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>