Use libc constants in sys/signal.rs.
Use libc constants in sys/signal.rs.

posborne/rust-nix,berkowski/nix,nikklassen/nix-rust,berkowski/nix,MarkusJais/nix-rust,Susurrus/nix,xd009642/nix,carllerche/nix-rust,kamalmarhubi/nix-rust,kamalmarhubi/nix-rust,geofft/nix-rust,Susurrus/nix,Susurrus/nix,nix-rust/nix,nix-rust/nix,geofft/nix-rust,asomers/nix,geofft/nix-rust,xd009642/nix,kamalmarhubi/nix-rust,posborne/rust-nix,xd009642/nix,MarkusJais/nix-rust,berkowski/nix,carllerche/nix-rust,nikklassen/nix-rust,MarkusJais/nix-rust,posborne/rust-nix,nikklassen/nix-rust,asomers/nix,carllerche/nix-rust