zfs: add an option to the bootloader to rewind the ZFS checkpoint

The checkpoints are another way of keeping the state of ZFS.
During the rewind, the pool has to be exported.
This makes checkpoints unusable when using ZFS as root.
Add the option to rewind the ZFS checkpoint at the boot time.
If checkpoint exists, a new option for rewinding a checkpoint will appear in
the bootloader menu.
We fully support boot environments.
If the rewind option is selected, the boot loader will show a list of
boot environments that existed before the checkpoint.

Reviewed by: tsoome, allanjude, kevans (ok with high-level overview)
Differential Revision: https://reviews.freebsd.org/D24920