conf-files: add flag so that we don't always prefix returned paths with the root dir path used

This is useful in tools such as system-repart where we show the
definition file paths a lot in our output, but if prefixed with the root
path we'd show a temporary mount dir when operating on a image file.
Hence, let's drop the prefix here, and show only the path within the
image.

src/repart/repart.c

@@ -3397,7 +3397,7 @@ static int context_read_definitions(Context *context) {
                         &files,
                         ".conf",
                         context->definitions ? NULL : arg_root,
-                        CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED|CONF_FILES_WARN,
+                        CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED|CONF_FILES_WARN|CONF_FILES_DONT_PREFIX_ROOT,
                         dirs);
         if (r < 0)
                 return log_error_errno(r, "Failed to enumerate *.conf files: %m");