tdb: introduce TDB_SUPPORTED_FEATURE_FLAGS

This will allow to store a feature mask in the tdb header on disk,
so that openers can check if they can handle the features
other openers are using.

Pair-Programmed-With: Volker Lendecke <vl@samba.org>
Pair-Programmed-With: Michael Adam <obnox@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Volker Lendecke <vl@samba.org>
Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>