This is meant to install a new BE (boot environment) given a fully built
world/kernel. In addition to installing world and kernel in the new BE,
it also automatically performs /etc updates (using etcupdate or mergemaster)
and package updates (using pkg).
Because this process is performed in a new BE, it reduces the need for a
second reboot. It also means a reboot into a partially updated system (due
to install or hardware failure) can't happen.
Inspired by and similar in function to Solaris/illumos-style upgrades.
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