cgls: add --machine/-M

cg_get_machine_path is modified to include the escaped machine name
+ ".nspawn" if the machine argument is nonnull.

src/test/test-cgroup-util.c

@@ -85,7 +85,7 @@ static void test_get_paths(void) {
         assert_se(cg_get_user_path(&c) >= 0);
         log_info("User = %s", c);
 
-        assert_se(cg_get_machine_path(&d) >= 0);
+        assert_se(cg_get_machine_path("harley", &d) >= 0);
         log_info("Machine = %s", d);
 }