checking @ATTRIBUTES attributes

[Andrew Tridgell]

Simo,

 > I've made a patch to check attribute flags in the special object
 > @ATTRIBUTES. To make things cleaner I also changed the way we store
 > flags in @ATTRIBUTES attributes by exploiting the fact the we support
 > multiple valued attributes in ldb.

yep, I think this is a good idea. I haven't tested that your patch
works, but the idea is fine, so if you've tested it sufficiently then
please apply.

 > +/* valid attribute flags */
 > +const struct {

minor nit - this should be a "static const struct"

also, we should add a ldb test that tries to set an invalid attribute
and see that it fails.

Thanks!