Add the ZFS test suite

It was originally written by Sun as part of the STF (Solaris test framework).
They open sourced it in OpenSolaris, then HighCloud partially ported it to
FreeBSD, and Spectra Logic finished the port. We also added many testcases,
fixed many broken ones, and converted them all to the ATF framework. We've had
help along the way from avg, araujo, smh, and brd.

By default most of the tests are disabled. Set the disks Kyua variable to
enable them.

Submitted by: asomers, will, justing, ken, brd, avg, araujo, smh
Sponsored by: Spectra Logic Corp, HighCloud