Update gpt.8:

Update gpt.8:
 * Add note that GPT partitions are accessed as DragonFly disk slices
 * Describe that add command will print disk slice.
 * Correct default partition type: it is dfly, not UFS
 * Mention that dfly partition type is for disklabel64.5
 * Add FILES section
 * Add EXAMPLES section
 * Add some Xrefs
 * Mention that migrate command isn'r for DragonFly
 * Add note that booting from GPT isn't supported