[zfsboot] Fix boot env back compat (#190)

[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 <ryan@freqlabs.com>