Don't allow two images to be split and tracked from a RAID LV at one time

Don't allow two images to be split and tracked from a RAID LV at one time

Also, don't allow a splitmirror operation on a RAID LV that is already tracking
a split, unless the operation is to stop the tracking and complete the split.
Example:
~> lvconvert --splitmirrors 1 --trackchanges vg/lv /dev/sdc1
# Now tracking changes - image can be merged back or split-off for good
~> lvconvert --splitmirrors 1 -n new_name vg/lv /dev/sdc1
# ^ Completes split ^

If a split is performed on a RAID that is tracking an already split image and
PVs are provided, we must ensure that
 1) the already split LV is represented in the PVs
 2) we are careful to split only the tracked image