file metadata via fs API (was: [GIT PULL] Filesystem Information)

[Al Viro]

On Wed, Aug 12, 2020 at 09:23:23AM +0200, Miklos Szeredi wrote:

> Anyway, starting with just introducing the alt namespace without
> unification seems to be a good first step. If that turns out to be
> workable, we can revisit unification later.

Start with coming up with answers to the questions on semantics
upthread.  To spare you the joy of digging through the branches
of that thread, how's that for starters?

"Can those suckers be passed to
...at() as starting points?  Can they be bound in namespace?
Can something be bound *on* them?  What do they have for inodes
and what maintains their inumbers (and st_dev, while we are at
it)?  Can _they_ have secondaries like that (sensu Swift)?
Is that a flat space, or can they be directories?"