Add tcsetpgrp function

[tinywombat765]

Could the tcsetpgrp function please be added? Thank you.

[posborne]

Certainly, would you mind taking a shot at implementing? I believe this one would be added to unistd.rs. Probably would want to have tcgetpgrp added as well.

Doing this would require two changes:

1. Add the raw FFI definitions to the libc crate. This would expose and unsafe interface.
2. Add functions that use these definitions to nix (with a safe interface).

If not, somebody here can probably get to it in time as well.

[tinywombat765]

Okay, I'm on it. Step 1 done.

[posborne]

Okay, I'm on it. Step 1 done.

Excellent, thanks for the update @zethra

[kamalmarhubi]

@zethra are you interested in adding it to nix as well? We can walk you through the process!

[tinywombat765]

@kamalmarhubi I've added them to my fork of nix. I'm just working on the documentation. But I'll happily except any help you're willing to give!

[kamalmarhubi]

Excellent! Feel free to open a PR from your fork and we'll take a look :-)

[kamalmarhubi]

I'm doing so poorly at keeping track. You already opened #451. Taking a look.

[Susurrus]

This was resolved by #451.