Detect busy status of Linux Software RAID members (#709640)

Read the contents of /proc/mdstat file to determine if a device is a
member of of an active RAID array.

    $ cat /proc/mdstat
    Personalities : [raid1]
    md1 : active raid1 sda1[2] sdb1[3]
          524224 blocks super 1.0 [2/2] [UU]

    md2 : active raid1 sdb2[2] sda2[3](F)
          5238720 blocks super 1.1 [2/1] [U_]

    md3 : active raid1 sdb3[1]
          10477440 blocks super 1.1 [2/1] [_U]
          bitmap: 1/1 pages [4KB], 65536KB chunk

    md4 : inactive sda4[0](S)
          1048564 blocks super 1.2

    unused devices: <none>

There are 5 example Linux Software RAID arrays, md1 to md5.  All are
RAID1 mirrors with 2 members, in various states.

    Array  Members     Status
    md1    sda1, sdb2  Fully operational.
    md2    sda2, sdb2  Member sda2 marked as faulty.  (Device sda2 is
                       still in use).
    md3    sda3, sdb3  Member sda3 has been removed.  (Device sda3 is
                       not in use).
    md4    sda4, sdb4  Incremental start of member sda4 only.  (Neither
                       member devices is in use).
    md5    sda5, sdb5  Array stopped.  (Neither member device is in
                       use).

Also disable "Unmount" in the partition menu for active RAID array
members.

Bug #709640 - Linux Swap Suspend and Software RAID partitions not
              recognised