Update gpt:

Update gpt:
 - note that DragonFly supports 32 slices per disk
 - increase size of boot slice (slice 0 made by boot command):
increase size from 128MB til 256MB, as total size of
kernel & modules w/ .old is over 128MB and
it is a hassle to increase size of boot slice later
 - add note that boot0 boot manager, which can be manipulated by
boot0cfg command, is used by boot command
 - add example of GPT boot disk setup
 - improve mark up a bit