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/nspawn/nspawn.c

@@ -4681,13 +4681,13 @@ static int load_settings(void) {
                  * actual image we shall boot. */
 
                 if (arg_image) {
-                        p = file_in_same_dir(arg_image, arg_settings_filename);
-                        if (!p)
-                                return log_oom();
-                } else if (arg_directory && !path_equal(arg_directory, "/")) {
-                        p = file_in_same_dir(arg_directory, arg_settings_filename);
-                        if (!p)
-                                return log_oom();
+                        r = file_in_same_dir(arg_image, arg_settings_filename, &p);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to generate settings path from image path: %m");
+                } else if (arg_directory) {
+                        r = file_in_same_dir(arg_directory, arg_settings_filename, &p);
+                        if (r < 0 && r != -EADDRNOTAVAIL) /* if directory is root fs, don't complain */
+                                return log_error_errno(r, "Failed to generate settings path from directory path: %m");
                 }
 
                 if (p) {