path-util: rework file_in_same_dir() on top of path_extract_directory()

Let's port one more over.

Note that this changes behaviour of file_in_same_dir() in some regards.
Specifically, a trailing slash of the input path will be treated
differently: previously we'd operate below that dir then, instead of the
parent. I think that makes little sense however, and I think the code
using this function doesn't expect that either.

Moroever, addresses some corner cases if the path is specified as "/" or
".", i.e. where e cannot extract a parent. These will now be treated as
error, which I think is much cleaner.

src/test/test-path-util.c

@@ -600,23 +600,19 @@ TEST(prefix_root) {
 TEST(file_in_same_dir) {
         char *t;
 
-        t = file_in_same_dir("/", "a");
+        assert_se(file_in_same_dir("/", "a", &t) == -EADDRNOTAVAIL);
+
+        assert_se(file_in_same_dir("/", "/a", &t) >= 0);
         assert_se(streq(t, "/a"));
         free(t);
 
-        t = file_in_same_dir("/", "/a");
-        assert_se(streq(t, "/a"));
+        assert_se(file_in_same_dir("", "a", &t) == -EINVAL);
+
+        assert_se(file_in_same_dir("a/", "x", &t) >= 0);
+        assert_se(streq(t, "x"));
         free(t);
 
-        t = file_in_same_dir("", "a");
-        assert_se(streq(t, "a"));
-        free(t);
-
-        t = file_in_same_dir("a/", "a");
-        assert_se(streq(t, "a/a"));
-        free(t);
-
-        t = file_in_same_dir("bar/foo", "bar");
+        assert_se(file_in_same_dir("bar/foo", "bar", &t) >= 0);
         assert_se(streq(t, "bar/bar"));
         free(t);
 }