fs-util: unify code we use to check if dirent's d_name is "." or ".."

We use different idioms at different places. Let's replace this is the
one true new idiom, that is even a bit faster...

src/basic/path-util.h

@@ -141,3 +141,5 @@ bool is_device_path(const char *path);
 bool is_deviceallow_pattern(const char *path);
 
 int systemd_installation_has_version(const char *root, unsigned minimal_version);
+
+bool dot_or_dot_dot(const char *path);