emulators/virtualbox-ose-additions: Add support for VirtualBox SharedFolder

This is an experimental implementation of VirtualBox SharedFolder subsystem support for FreeBSD.
The implementation is based on github.com/lwhsu/freebsd-vboxfs.
It's tested and worked as expected.
The locking mechanism may need enhancements.

Reviewed by: jkim (vbox), mat (mentor)
Approved by: jkim (vbox), mat (mentor)
Sponsored by: Netzkommune GmbH
Differential Revision: https://reviews.freebsd.org/D11602