Unbreak the installer on head/:

  When bsdinstall(8) sources the bsdconfig(8) common.subr file,
  PKG_ABI is set by calling 'pkg -vv' and searching for the ABI
  pkg(8) will use.

  When pkg(8) is run for the first time, the bootstrap process
  is run, which prompts for 'y/N' input from stdin if running with
  TERM set.

  Since TERM is set and it is the first time pkg(8) is run, which
  happens automatically, bsdinstall(8) hangs waiting for user input
  which is never displayed since a specific line is expected by
  awk(1), and stdin is expected by pkg(8).

  Set ASSUME_ALWAYS_YES=1, which will cause pkg(8) to assume the
  '-y' flag is also used for the bootstrap process, allowing
  bsdinstall(8) to proceed to the keymap lookup, otherwise
  bsdinstall(8) appears to hang after selecting 'Install' from the
  menu on first boot from CDROM.
Remove the env(1) but keep the var.
Check the partition scheme before blowing away disks, instead of after.
The effects of this patch would only be noticeable if you were purposefully
setting a bad value and trying to see what happens; and leaving the disks
intact if a bad value has been set seems fair.