zfs boot manual pages

I've started working on manual pages for the zfs boot chain.

Please [p]review my work in progress here:
https://github.com/avg-I/freebsd/compare/review;zfs-boot-man-pages

Any additions, corrections, suggestions and other kinds of reviewing are
welcome.  Patches and pull requests are very welcome!

Many thanks to Warren Block for the initial review and many fixes.
-- 
Andriy Gapon