clean-up: Rename lvm.conf setting 'mirror_region_size' to 'raid_region_size'

clean-up:  Rename lvm.conf setting 'mirror_region_size' to 'raid_region_size'

We have been using 'mirror_region_size' in lvm.conf as the default region
size for RAID logical volumes as well as mirror logical volumes.  Since,
"raid" is more inclusive and representative than "mirror", I have changed
the name of this setting.  We must still check for the old setting and warn
the user if we are overriding it with the new setting if both happen to be
present.