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.
This gives auto-cancelling semantics on `Drop`, plus a nicer
`.commit()` method on the transaction.
Matches the currently private `_OstreeRepoAutoTransaction` in the C
library.
An intimidating spam of compiler errors at the start, but the
biggest was handling the new convention of `ostree_sys::` => `ffi::`.
This will require a semver bump of course.
Colin Walters
Felix Krull