zfs: bring back possibility to rewind the checkpoint from
Add parsing of the rewind options.
When I was upstreaming the change , I omitted the part where we
detect that the pool should be rewind. When the FreeBSD repo has
synced with the OpenZFS, this part of the code was removed.
Originally reviewed by: tsoome, allanjude
Originally reviewed by: kevans (ok from high-level overview)
Signed-off-by: Mariusz Zaborski <firstname.lastname@example.org>