Detect Linux SWRaid members by querying mdadm (#756829)

Detection of Linux SWRaid members currently fails in a number of cases:

1)  Arrays which use metadata type 0.90 or 1.0 store the super block at
    the end of the partition.  So file system signatures in at least
    linear and mirrored arrays occur at the same offsets in the
    underlying partitions.  As libparted only recognises file systems
    this is what is detected, rather than an SWRaid member.

    # mdadm -E -s -v
    ARRAY /dev/md/1  level=raid1 metadata=1.0 num-devices=2 UUID=15224a42:c25bbcd9:15db6000:4e5fe53a name=chimney:1
       devices=/dev/sda1,/dev/sdb1
    ...
    # wipefs /dev/sda1
    offset               type
    ----------------------------------------------------------------
    0x438                ext4   [filesystem]
                         LABEL: chimney-boot
                         UUID:  10ab5f7d-7d8a-4171-8b6a-5e973b402501

    0x1fffe000           linux_raid_member   [raid]
                         LABEL: chimney:1
                         UUID:  15224a42-c25b-bcd9-15db-60004e5fe53a

    # parted /dev/sda print
    Model: ATA VBOX HARDDISK (scsi)
    Disk /dev/sda: 34.4GB
    Sector size (logical/physical): 512B/512B
    Partition Table: msdos

    Number  Start   End     Size    Type      File system  Flags
     1      1049kB  538MB   537MB   primary   ext4         boot, raid
    ...

2)  Again with metadata type 0.90 or 1.0 arrays blkid may report the
    contained file system instead of an SWRaid member.  Have a single
    example of this configuration with a mirrored array containing the
    /boot file system.  Blkid reports one member as ext4 and the other as
    SWRaid!

    # blkid | egrep 'sd[ab]1'
    /dev/sda1: UUID="10ab5f7d-7d8a-4171-8b6a-5e973b402501" TYPE="ext4" LABEL="chimney-boot"
    /dev/sdb1: UUID="15224a42-c25b-bcd9-15db-60004e5fe53a" UUID_SUB="0a095e45-9360-1b17-0ad1-1fe369e22b98" LABEL="chimney:1" TYPE="linux_raid_member"

    Bypassing the blkid cache gets the correct result.

    # blkid -c /dev/null /dev/sda1
    /dev/sda1: UUID="15224a42-c25b-bcd9-15db-60004e5fe53a" UUID_SUB="d0460f90-d11a-e80a-ee1c-3d104dae7e5d" LABEL="chimney:1" TYPE="linux_raid_member"

    However this can't be used because if a user has a floppy configured
    in the BIOS but no floppy attached, GParted will wait for minutes as
    the kernel tries to access non-existent hardware on behalf of the
    blkid query.  See commit:
        18f86315
        Fix long scan problem when BIOS floppy setting incorrect

3)  Old versions of blkid don't recognise SWRaid members at all so always
    report the file system when found.  Occurs with blkid v1.0 on
    RedHat / CentOS 5.

The only way I can see how to fix all these cases is to use the mdadm
command to query the configured arrays.  Then use this information for
first choice when detecting partition content, making the order: SWRaid
members, libparted, blkid and internal.

GParted shell wrapper already creates temporary blank udev rules to
prevent Linux Software RAID arrays being automatically started when
GParted refreshes its device information[1].  However an administrator
could manually stop or start arrays or change their configuration
between refreshes so GParted must load this information every refresh.
On my desktop with 4 internal hard drives and 3 testing Linux Software
RAID arrays, running mdadm adds between 0.20 and 0.30 seconds to the
device refresh time.

[1] a255abf3
    Prevent GParted starting stopped Linux Software RAID arrays (#709640)

Bug 756829 - SWRaid member detection enhancements