Pidfd enhancements

- [x] rust-lang/libc#4402
- [ ] `PidfdFlags::THREAD = PIDFD_THREAD`
- [x] `pidfd_send_signal` (#992) is in `rustix::not_implemented::yet` and `rustix::process` (#1448)
- [ ] `PIDFD_SIGNAL_THREAD`, `PIDFD_SIGNAL_THREAD_GROUP` and `PIDFD_SIGNAL_PROCESS_GROUP`.
- Because the `rustix::process::pidfd_send_signal` interface does not expose an (empty) flags argument and adding one would be a breaking change, I suggest to deprecated `rustix::process::pidfd_send_signal` and add `rustix::process::pidfd_send_signal_{thread,thread_group,process_group}` variants. This time with an empty flags for the future.
- On the other hand this could require downstream code to switch between these variants to maybe a `rustix::process::pidfd_send_signal_with_flags` (name tbd) is also a thing.
- `pidfd_send_signal_to`
- [ ] `PIDFD_SELF` and `PIDFD_SELF_PROCESS` descriptors implemented in `rustix::process` like `rustix::fs::CWD` is implemented.
- [ ] `PIDFD_GET_*_NAMESPACE` ioctls. Either as one function with an non-exhaustive enum argument or as separated for functions for each ioctl.
- [ ] `PIDFD_GET_INFO` ioctl, `pidfd_info` struct, `PIDFD_INFO_*` bitflags
- [ ] `PIDFD_INFO_EXIT`