Draft: init.d/parted: Skip installation media

Partman should not touch the installation media partition table, because it can cause unexpected issues with custom isohybrid images in EFI mode. Currently, it only works thanks to quirks in official isohybrid images and libparted but we should not rely on it.

Also the user should not be allowed to modify partitions on the installation media, in order to avoid mistakes.

So just skip the device if it or one of its partitions is mounted on /cdrom or /hd-media.