post_status API improvements

[filosofo]

There's an inchoate API, but:

• It's not being used to generate the shown options when editing a particular post object
• The API functions have copy-paste cruft, such as the "hierarchical" property for post_status
• It would be reasonable to associate status types with particular object types, something you can't do currently.

One problem is that many status types are extensively hard-coded into WP.

[scribu]

One problem is that many status types are extensively hard-coded into WP.

Going slightly offtopic, I would like to point out that it's the same situation (maybe worse) for comment statuses.

[ptahdunbar]

This looks like a dup of #12706