hakurei 0.3.1

Fixes:
- Format group lookup error message correctly.
- Reject impossible user-supplied fd numbers.
- Close initial process extra files on termination, which also keeps them alive
  until then.
- Close `container/init` setup pipe before fork/exec of initial process.

Enhancements:
- Expose current instance identifier via the `--identifier-fd` flag.
- Set FD_CLOEXEC on files left open by the parent.

Internal:
- Separate seccomp numbers from syscall numbers to prevent inadvertent use of PNRs.
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