beinstall: perform pre-installworld steps.

Since all post-installkernel steps are assumed to operate in the updated
installation, it's necessary to chroot all of the followup steps in the new
boot environment. Set up and mount the source and object directories at the
same paths inside the BE root, and clean up to the extent changes were made.
This commit fixes upgrading using beinstall past the new ntpd user change.

Improve testability of changes to this script while I'm here.

Reported by: rpokala (earlier patch)