zv: Implement `io_lifetimes::AsFd` and `From` conversion with `OwnedFd`

This provides at least safe interop with standard IO safety types, to the extent that's practical without breaking changes (#286).

For Fd this is more problematic since that lacks a lifetime and cannot guarantee it contains a valid fd.

In recent Rust version, this is a re-export of the std type. When zbus depends on a Rust version that has this, the dependency can be dropped.