Use makefs(8) in release VM-image generation instead of mdconfig/newfs/etc.

[nwhitehorn]

Summary

This patch changes release VM image generation to use makefs(8) instead of using md(4), then newfs, and mounting and unmounting things many times. This provides a number of benefits:

• Simpler generation of images (the patch is net-negative): no need to attach md(4) devices and mount and unmount things
• Reduced privileges needed: if not for needing to have devfs mounted in the image at a few points, could do this in a zero-privilege jail
• Can generate cross-endian file systems: this enables generation of powerpc64 VM images (tested and working) on amd64 hosts

I have tested generation of amd64 and powerpc64 images and they work as before, with identical file system parameters and organization.

[gjb]

No objection, but please do not target this for releng/13.0 (13.0-RELEASE), as it is my opinion it is too late for such a change at this point in the cycle.

[nwhitehorn]

Just committed this. I agree about 13.0 -- I was planning to merge to stable/13 after 13.0 has branched and smoke-test this in HEAD for a bit in the meantime.