Add jail functions and constants for FreeBSD

[ghost]

These are the system calls for creating and managing jails on FreeBSD.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 7c03711 has been approved by alexcrichton

[bors]

⌛ Testing commit 7c03711 with merge 0824426...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 0824426 to master...