spl, zfs: Add git versions, based on recent commits

Upstream has not been tagging new versions for a long time, but we need
compatibility with newer kernels. The 0.6.2 versions already have a bunch of
backported compatibility patches, but 3.14 kernels need even more.

Also, the git versions have fixed a bunch of crashes and other bugs, so perhaps
we should just bite the bullet and just use recent git versions (as sometimes
upstream recommends, when people run into bugs).

This adds a new "boot.zfs.useGit" boolean option, so that a user can
easily opt into using the git versions.