If logical volumes are active and recognised by udev, no longer ignore
them. We also do some extra handling to use user-friendly device names
and mount the /dev/mapper/foo device rather than the /dev/dm-xx device.
Additionally if we see "LMV2_member" devices start a rescan in case
LVM-formatted disks came up after the LVM initscript.
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>