geom_part: support < 128 GPT partition entries

Hi,

I'd appreciate a review for: https://reviews.freebsd.org/D6900

Summary:
This change is useful primarily for using GPT on embedded boards like the
pine64 (and others using an Allwinner SoC) that want a firmware image loaded
below sector 34 (8K in that instance).

Note that because this changes g_part_scheme, it breaks the ABI.

Thanks!
-- 
wca