Fish: useOperatingSystemEtc should load conf.d snippets and functions as well as config.fish

[therealpxc]

Describe the bug

In our Fish package, the sysconf dir is points to the Nix store, since Fish, like everything, gets its own unique prefix. However, to meet user expectations, we have an option when building the Fish package to have it read from the normal operating system configuration directories (/etc/fish) as well. Unfortunately, this setting only plugs into the built-in sysconfdir's etc/fish/config.fish file, and doesn't load paths in /etc/fish/conf.d or /etc/fish/functions correctly.

I would like to find time to fix this bug in the next day or two. :)