* configure.ac: Check for Linux device-mapper support.
* util/hostdisk.c (device_is_mapped): New function.
(find_partition_start): New function, partly broken out from
linux_find_partition and grub_util_biosdisk_get_grub_dev but with
device-mapper support added.
(linux_find_partition): Use find_partition_start.
(convert_system_partition_to_system_disk): Add `st' argument.
Support Linux /dev/mapper/* devices if device-mapper support is
available; only DM-RAID devices are understood at present.
(find_system_device): Add `st' argument. Pass it to
convert_system_partition_to_system_disk.
(grub_util_biosdisk_get_grub_dev): Pass stat result to
find_system_device and convert_system_partition_to_system_disk. Use
find_partition_start.
