redox: use *at and dirent functions

[xtqqczze]

See rust-lang/libc#5117

[willnode]

There's already a draft for that in #1638

[joshuamegnauth54]

Sorry for the delay. 😁 I'm waiting on libc version bumps.

Ideally, I'd like to unmask as much as I can in libc, nix, and rustix so that downstream crates have less feature gates.