Handle relative paths that are relative to $(dirname "$0")

In tor-config we handle CfgPath instances that are relative to the home dir, the state dir, and the cache dir.

We should additionally handle paths that are relative to the arti binary; these will help with tools like gosling and tor browser that want to ship pluggable transports.

It's an open question whether we should resolve symlinks before or after looking up these paths.

This is something @richard would like, and probably others would too.