linux: recognize md devices again, but know they can't be partitioned

linux: recognize md devices again, but know they can't be partitioned

* libparted/arch/linux.c (_skip_entry): Don't skip "md" devices.
(_has_partitions): New function; "loop" labels can't be partitioned.
(_blkpg_add_partition, _blkpg_remove_partition, _dm_add_partition,
_have_blkpg): Don't attempt to apply partition table changes to
unpartitionable devices.