MFC r348569: [zfsboot] Fix boot env back compat (#190)
- Fix boot env back compat
zfsboot must try zfsloader before loader in order to remain compatible
with boot environments created prior to zfs functionality being rolled
into loader proper.
- Improve comments in zfsboot
Explain the significance of the load path order, and put the comment
about looping through the paths in the appropriate scope.
Approved by: re (gjb)