Add a `cap-std-apis` feature with open/create

Add a `cap-std-apis` feature with open/create

I'm trying to make more use of `cap-std` in our stack, and
this will be a key enabling API.

Actually a notable side benefit of this is that we don't need
to teach the ostree C code itself to use `openat2`, we inherit
cap-std's setup.

All of the internal ostree code using the prior `openat()` should
continue to work.

I only did basic sanity checking of this; there may be bugs
in other APIs.