OSSL_STORE: Treat URIs as files first (with exceptions), then as full URIs

To handle paths that contain devices (for example, C:/foo/bar.pem on
Windows), try to "open" the URI using the file scheme loader first,
and failing that, check if the device is really a scheme we know.

The "file" scheme does the same kind of thing to pick out the path
part of the URI.

An exception to this special treatment is if the URI has an authority
part (something that starts with "//" directly after what looks like a
scheme).  Such URIs will never be treated as plain file paths.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3907)