This is a follow-up of #10717 to use the same naming convention as on
Linux guests, in order to reduce the diffs.
Also adds the missing capability to `unmount_virtualbox_shared_folder`
on FreeBSD guests.
BSD-based guests do not support VirtualBox shared folders. This is a
common source of confusion in Vagrant. This new error clearly explains
that this is not a bug in Vagrant and provides instructions on how to
disable them.