Resync thermite with the re@ version:

• thermite.sh:
  • Add support for aarch64 EC2 AMIs.
  • Move creating seed src checkouts for amd64 and i386 to zfs_mount_src().
  • Create a zfs dataset for the amd64 and i386 userland which later is snapshotted and cloned, eliminating multiple invocations of the installworld target.
  • Explicitly return 0 in build_chroots().
  • Make use of ZFS a requirement.
  • Update and sort Copyright.

zfs-setup.sh:

• Rename to zfs-cleanup.sh.
• Default to cleanup-only. ZFS datasets are now created via thermite.sh.

This is the first batch of changes in preparation of adding support
for snapshot/release builds from both git and svn.

Sponsored by: Rubicon Communications, LLC (netgate.com)