Provide more robust support for Mac OS X and lack of VirtualBox support for guest control additions.

If Mac OS X is detected, request the user if they really want to reuse VM images, since the automatic
installation of newer browsers cannot be done.  The user can still install the browser version directly,
though it's more cumbersome.  In any case, this PR fixes issues with guest controls not working correctly
on Mac OS X and causing the script to hang during installs.