raid: add reshape segtype flag support

raid: add reshape segtype flag support

Prohibit activation of reshaping RaidLVs on incompatible
lvm2 runtime by storing e.g. 'raid5+RESHAPE' segment type
strings in the lvm2 metadata.  Incompatible runtime not
supporting reshaping won't be able to activate those thus
avoiding potential data corruption.

Any new non-reshaping lvconvert command will reset the
segment type string from 'raid5+RESHAPE' to 'raid5'.

See commits
0299a7af1ec1eeb11388ed15c4c78f224fee76b2 and
4141409eb09e78aef030346995f1722fb40956d8
for segtype flag support.