A hybrid approach was previously employed that made it possible
to use admin as a synonym for root, which was really confusing
and a bit unclear judging by the actual code employed.  Does
admin own a home directory or not?  Why does root work on the
console, but not in the web interface?

Also reapply a few after recent merge/rebase action in the CP.

Looks like factory_shipped_username was never really honoured here.
Clean this up while bringing general sanity to the file.