Reenter via exec (ditch the extra shell)
Remove PLUSHU_SCRIPT parameter

The layout of PLUSHU_ROOT isn't really up for configuration like that
Use extended tests in install.sh
Don't specify the plugins to delete in the fallback message

Granted, now we're telling the user to nuke any plugins they
originally had installed, which isn't really a great solution,
either, but at least it's easier to maintain, if we're sticking
to the right-after-installation scope.