zfsbootcfg(8): Rewrite to use libbe

[kevans]

Summary

(This will get committed as two separate parts, but both are included in this review for easier review)

Instead of relying on the boot path to setup the kenv, use libbe which should divine the same information from the dataset booted at / and scurrying through ZPOOL_CONFIG_VDEV_TREE.

This is completely untested, but in theory it should work.

[kevans]

This needs a little bit more consideration -- it would fail miserably if operating on a system that doesn't have a traditional BE setup or the dataset mounted at / isn't at least two levels deep.

[avg]

Or if a "boot pool" setup is used (kernel is loaded from one pool, root is mounted from another). Or if reboot -r has been run prior to zfsbootcfg.

[bcr]

Ok for the man page part of the change.

[kevans]

In D19008#406325, @avg wrote:

Or if a "boot pool" setup is used (kernel is loaded from one pool, root is mounted

And /boot isn't guaranteed to be remounted after reroot, is it, given that it's not really required if everything could be preloaded prior?

[kevans]

Abandoning in favor of D19019 to start setting up zfsbootcfg vars in loaders properly; I think zfsbootcfg should likely bail out if machdep.bootmethod is unset or != "BIOS" until we teach other loaders pad2 bits, though.