eventfd: Follow nix conventions
eventfd: Follow nix conventions

This commit revamps to eventfd to follow nix conventions:
  - drop in-crate FFI definitions
  - rename EventFdFlag to EfdFlags

Additionally, it changes the initval argument to be a libc::c_uint,
matching the actual type.

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