Add prctl(2) support

[cyphar]

I'm working on an alternative container init and signal forwarder, and a nice feature would be to be able to run it as non-pid1. However, that requires being able to set PR_SET_CHILD_SUBREAPER from within Rust.

While it's possible to do this:

use libc::{prctl, PR_SET_CHILD_SUBREAPER};
unsafe { prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0); }

The unsafe is unsightly.

[Susurrus]

Indeed, it seems reasonable to add prctl(2). Would you be willing to submit a PR for this?

[cyphar]

I'm not a very proficient user of Rust yet (I just started picking it up last weekend), but I believe that the prctl crate could be a reasonable starting point. The biggest thing that makes prctl(2) ugly and unsafe is that the arguments are sometimes read and sometimes written to by the kernel.

[Susurrus]

If you want to have a go at it, I'll mentor you. This crate does very low level abstractions, so basically what you already wrote puts you half way there I'd think. We wouldn't try to offer a more comprehensive high-level API as that crate seems to be aiming for. Otherwise I don't know how likely it is for someone to tackle this in the near future.

[SteveLauC]

Close as completed in #1550