repo: Add two more cap-std APIs

repo: Add two more cap-std APIs

Followup to the previous PR.  I realized now with `io_lifetimes`
we can offer a safe `dfd_borrow()` that *borrows* the file descriptor
for the repository.  (In contrast to the current `.dfd()` that returns
the raw version)

Building on that, add another API that re-acquires a `Dir` instance.
(In the future in theory we could optimize this more by knowing
 whether or not the repo was constructed via cap-std, and perhaps
 in theory synthesize a `&Dir` reference, but I don't think we
 need that now)