FreeBSD: bring back possibility to rewind the checkpoint from bootloader

Add parsing of the rewind options.

When I was upstreaming the change [1], 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.

[1] FreeBSD repo: 277f38abffc6a8160b5044128b5b2c620fbb970c
[2] OpenZFS repo: f2c027bd6a003ec5793f8716e6189c389c60f47a

External-issue: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254152
Originally reviewed by: tsoome, allanjude
Originally reviewed by: kevans (ok from high-level overview)
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mariusz Zaborski <oshogbo@vexillium.org>
Closes #11730