systemctl: fix broken list-unit-files with --root

This patch modifies unit_file_get_list which will now return
hashmap of structures where f->path is *without* root_dir prefix.

This change should be ok, because current code either does not use
root_dir at all or calls basename() on the f->path.

src/shared/install.c

@@ -2099,7 +2099,7 @@ int unit_file_get_list(
                         if (!f)
                                 return -ENOMEM;
 
-                        f->path = path_make_absolute(de->d_name, units_dir);
+                        f->path = path_make_absolute(de->d_name, *i);
                         if (!f->path)
                                 return -ENOMEM;