Make Errno::result() inline
Make Errno::result() inline

This makes the success case a single `cmpl $-1, %eax` followed
with a non-taken conditional jump, instead of the current `callq`
which is always taken.

I have not benchmarked the difference, but it is an obvious
improvement.
nix-rust/nix,nix-rust/nix,asomers/nix,asomers/nix