Add LDB_MAP_RENDROP option

Add LDB_MAP_RENDROP option

Like LDB_MAP_RENAME, but drop the attribute if it occurs in an Add request.
Used for distinguishedName attribute, is read-only and generated but for
some bizarre reason AD allows it in an Add request.

Signed-off-by: Howard Chu <hyc@symas.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Nadezhda Ivanova <nivanova@symas.com>