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.
This also accomplishes the following:
- Proxy through zfs_nicenum as be_nicenum, because it looks better than
humanize_number and would presumably be useful to other libbe consumers.
- Rename be_get_snapshot_props to be_get_dataset_props, make it more useful
Based on the idea that we shouldn't have all-new library and utility going
into base that need WARNS=1...
- Decent amount of constification
- Lots of parentheses
- Minor other nits
This makes us more resilient to a rename of the bootfs, but still wouldn't
withstand pool renames or guid renames. More importantly, this allows `bectl
create <foo>` work out of the box to create a boot environment based on the
currently booted one.
- Rename 'active' to 'rootfs', which is used in other places to describe the
currently booted (or about to be booted) BE.
- Add 'bootfs', which indicates the next boot environment to be booted. This
is pulled from the BOOTFS zpool property.
- Go ahead and keep an open handle to the active zpool. We might need to
enumerate datasets, get properties, and set properties (e.g. bootfs)
throughout other libbe bits, and a single handle isn't overly expensive.
