locate service: allow customisation

Fixes #2961

It'd be great if someone could spend 2 mins eyeing this for any obvious mistakes.

I note that `nixos-option` does not show updated values, I don't know what the deal with that is.
The optionalString shouldn't be necessary, just (concatStringsSep " " cfg.extraFlags) should be fine.
Also does it work with empty extra flags? I'm worried about that trailing backslash.