Add IPv6 support

[grembo]

This aims to detect an IPv6 only system and does what's necessary to allow accessing mfsBSD.

Also:

• add missing dependency on kvm-ok
• add monitor socket to qemu (useful for debugging)

This addresses #10, which I can test for a full install, once this and https://github.com/depenguin-me/depenguin-builder/pull/9 landed.

[bretton]

needs a version bump