vg_validate: segment check enhancements for raids to catch bogus metadata

vg_validate: segment check enhancements for raids to catch bogus metadata

General RAID and RAID segment type specific checks are added
to merge.c. New static _check_raid_seg() is called on each segment
of a RaidLV (which have just one) from check_lv_segments().

New checks caught some unititialized segment members
which are addressed here as well:

- initialize seg->region_size to 0 in lvcreate.c for raid0/raid0_meta

- initialize list seg->origin_list in lv_manip.c