chase: check root path in more detail

In chaseat() we call dir_fd_is_root() several places, and the final
result depends on it. If the root path specified to `chase()` is not
normalized but points to "/", e.g. "/../", assertions in `chaseat()` or
`chase()` may be triggered.

src/test/test-chase.c

@@ -73,6 +73,11 @@ TEST(chase) {
         assert_se(path_equal(result, "/usr"));
         result = mfree(result);
 
+        r = chase(p, "/.//../../../", 0, &result, NULL);
+        assert_se(r > 0);
+        assert_se(path_equal(result, "/usr"));
+        result = mfree(result);
+
         pslash = strjoina(p, "/");
         r = chase(pslash, NULL, 0, &result, NULL);
         assert_se(r > 0);