libbe(3): Rewrite activate temp bits to rely less on loader

libbe(3): Rewrite activate temp bits to rely less on loader

Loader is still relied upon at the beginning of libbe to specify the be
root, but we can derive from that the primary zpool and any vdevs that we
need to set nextboot bits on.

This lets me successfully `bectl activate -t test`, but UEFI loader doesn't
quite yet understand so it's effectively defunct.