do not panic when do not have perms to /etc/{protocols,services}

This may happen when running as a less privileged user or in a sandbox that does not hide the existence of the file but denies access to it. Specifically, this is done to fix a Nix build on macOS with sandboxing enabled that denies access to these files.

Presumption here is that the original intent was to panic only on “unusual” I/O errors where the file was readable.