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/basic/conf-files.c

@@ -539,7 +539,7 @@ static int copy_and_sort_files_from_hashmap(
 
                 if (FLAGS_SET(flags, CONF_FILES_BASENAME))
                         add = c->filename;
-                else if (root) {
+                else if (root && !FLAGS_SET(flags, CONF_FILES_DONT_PREFIX_ROOT)) {
                         _cleanup_free_ char *p = NULL;
 
                         r = chaseat_prefix_root(c->result, root, &p);