Auto merge of #347 - fiveop:sigset_enhancement, r=kamalmarhubi
Auto merge of #347 - fiveop:sigset_enhancement, r=kamalmarhubi

Add SigSet::union and SigSet::clear.

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