Prefer mountpoints to use the Block IdLabel for the filesystem label

The commit messages are self-explanatory. Also added some comments to mention the reason for the changes.

I really think this helps FreeBSD look more "desktop-friendly" when users see their devices with filesystem labels and have mounted /media/<device> paths using them. For example when using x11-fm/dolphin. If the filesystem label is not found, then fallback to the original behavior. 😎