Should maybe clarify that RLIM_INFINITY comes from libc

[hlavaatch]

trafficstars

Doc for getrlimit and setrlimit mentions RLIM_INFINITY constant but doesn't mention it comes from the libc crate. Maybe it should, or the constant could be re-exported in the nix::sys::resource namespace. As it is now, search for RLIM_INFINITY in nix doc yields nothing, leaving user scratching his head :)

[asomers]

Good point. Reexporting it makes sense, I think. Would you care to submit a PR for that?