tor-config expand-paths feature is not (quite) additive

With this feature, $ in paths is interpreted by shellexpand.

Without it, it's a literal.

In crates/tor-config/src/path.rs, the #[cfg(not(feature = "expand-paths"))] fn expand... ought to:

• Check for $
• Implemewnt some minimal escaping syntax that is compatible with shellexpand so that there is a way to specify a literal $ that works in both configurations
• Reject other uses of $ with a message about the feature being disabled

Prompted by my review of !1068 (closed)