dsdb:replmd: add compatible feature helper function

dsdb:replmd: add compatible feature helper function

repl_meta_data.c uses the compatible features attribute of the
"@SAMBA_DSDB" special object to record that linked attributes are
being stored in the database in a sorted order. Soon the
linked_attributes module is going to want to know the same thing, and
in time other modules will want to know about other compatible
features, so we introduce a helper function.

Error checking is slightly improved.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>