Pidfd enhancements

[rusty-snake]

• [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