Implement pidfds

[copybara-service]

Implement pidfds

This patch introduces pidfd support roughly up to Linux 6.8. Linux selftests
at this version pass, and are now run as an //image_test target. Some features,
like PIDFD_THREAD, that occur later than 6.8 in Linux, are included.

It includes:

• These new syscalls:
  • pidfd_open()
  • pidfd_getfd()
  • pidfd_send_signal()
• New aspects to these existing syscalls:
  • waitid() in its support of P_PIDFD
  • clone()/clone3() in their support of CLONE_PIDFD
  • poll()/epoll()/select() in their support for pidfds
  • setns() in its support for pidfds
  • pid introspection via /proc/[pid]/fdinfo/[pidfd]

It excludes:

• pidfdfs: pidfds are backed by the anon inode right now
• ioctls to retrieve namespace fds using a pidfd
• process_madvise()

Updates #5839.