Access using psz_filepath for non-file URI

Historically, the core would URI-decode the "path" (everything after the MRL scheme) and pass it in psz_path. This broke some access plugins, so we distinguished the full URI psz_url, the URL minus scheme psz_location and the local file path psz_filepath.

There was no proper plan for plugins using non-file MRLs but expected a decoded/local path. This becomes a real problem after !633 (merged)