[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.
Obtained From: TrueNAS commit 4c60c62fcf0b6b6eac98ee8d46e7bbea64bc86f5
Submitted by: Ryan Moeller <email@example.com>