Copying /etc/resolv.conf to chroot while restoring the original file unless it has been changed during provisioning

[chrsoo]

This PR relates to #144.

There are very likely room for improvements both in algorithm and code, please review carefully before merging!

Change tested on Macbook Pro M1 with ubuntu-20.04.2-preinstalled-server-arm64+raspi.img.xz and confirmed working.

[dbast]

This are 123 lines of added code... how is it fundamentally different from, e.g.?

  provisioner "shell" {
    inline = [
      "mv /etc/resolv.conf /etc/resolv.conf.bak",
    ]
  }
  provisioner "file" {
    source      = "/etc/resolv.conf"
    destination = "/etc/resolv.conf"
  }
  provisioner "shell" {
    inline = [
      "rm -rf /etc/resolv.conf",
      "mv /etc/resolv.conf.bak /etc/resolv.conf",
    ]
  }

Why implement this on the go-side inside the builder? Above solution can also be customised, by e.g. skipping the restore etc with not unexpected automatic behaviour.