Auto merge of #285 - fiveop:use_libc, r=@kamalmarhubi
Auto merge of #285 - fiveop:use_libc, r=@kamalmarhubi

Use libc constants in sys/signal.rs.

Work toward #264.

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