emulators/virtualbox-ose: add option to build vboximg-mount

Introduce new port option VBOXIMG that enables building and installing
"vboximg-mount" utility. It is not enabled by default as it needs
fusesfs-libs as another dependency.

The utility gives access to insides of VM images like .vdi files etc.
Use "vboximg-mount -g" to get access to files on internal file systems
not supported by native FreeBSD mount command. Refer to utility's
internal online help for syntax details and to original documentation:

https://docs.oracle.com/en/virtualization/virtualbox/6.0/user/vboximg-mount.html

PR: 274817
Tested by: vvd
Approved by: vbox (maintainer timeout over 2 weeks)