kboot: Add ZFS support to hostdisk

[imp]

Summary

Add helper function to walk through the disk drives we've found to look
for zpools. main.c will still need to call this because the loader
hasn't implemented a good way to 'taste' drives for zpools and/or GELI
partitions (mostly because there's no generic list of candidate
devices).

Sponsored by: Netflix

[imp]

stand/kboot/hostdisk.c

477

... or at least somewhere in common since it needs sanity_check_currdev and It's not clear to me that belongs in libsa instead of common, which means probe_zfs_currdev shouldn't be in libsa since it would then depend on something defined in common...

508

I moved this here to make sure that we only setup this stuff if we're successful. It's not clear the error handling is quite right here though. But init_zfs_boot_options also has some error handling issues if it bails out in the middle: It will leave a number of env vars set improperly (imho).

[imp]

rebase / ping