gpt - Add a new directive called 'init'

gpt - Add a new directive called 'init'

* Add a new directive called 'init'.  This directive works like
  gpt create except it will also create a msdos slice 0 (and newfs_msdos
  it), and a "DragonFly Label64" slice 1, and
  'disklabel -r -w <slice1> auto'.

* If -B is also specified, the init directive will populate the msdos
  filesystem with efi/boot/bootx64.efi, and also supplies -B to the
  disklabel command.

* This command does not create the boot, swap, or root filesystem
  partitions in the disklabel.  It is meant to do something similar
  to what fdisk -IB used to do for us for legacy partitions (though
  I should note that fdisk -IB didn't initialize a new disklabel).  But
  we do for gpt.