gpt.8: Update example and note

gpt.8: Update example and note

 * don't use da8 as general disk in examples,
   as it is now default for removable device

 * adjust boot setup example to present defaults:
   * no `-o packet' needed for bootcfg,
   * kernel lives in boot/, no need for extra mv

 * use preferred HAMMER label (as in installer & rconfig example)

 * add suggestion for exact text to commands and break too long lines

 * drop note that boot only is supported on 32 bit disklabels