lvconvert: add infrastructure for RaidLV reshaping support
In order to support striped raid5/6/10 LV reshaping (change
of LV type, stripesize or number of legs), this patch
introduces more local infrastructure to raid_manip.c
used by followup patches.
Changes:
- add missing possible reshape conversions and conversion options
to allow/prohibit changing stripesize or number fo stripes
- enhance setting convenient riad types in reshape conversions
(e.g. raid1 with 2 legs -> radi5_n)