epoll: Use libc_bitflags for EpollFlags
epoll: Use libc_bitflags for EpollFlags

This is possible now that EPOLLEXCLUSIVE was added to libc in
https://github.com/rust-lang/libc/pull/526

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