System Update utility

Write a __System Update__ utility that installs the update into a new Boot Environment.

Along the lines of

```
pkg upgrade --fetch-only -y
beadm list
beadm create c572e
beadm mount c572e /media/c572e
chroot /media/c572e
pkg upgrade -y
pkg upgrade -r FreeBSD -y
exit
beadm list
beadm umount c572e
```

Thanks @grahamperrin

We could then make the next boot go into the new Boot Environment, and ask the user there whether all subsequent boots should continue to go there. Or something like that.