Migrate (mostly) from parted to sgdisk

[Lanchon]

sgdisk would allow retention of much more GPT metadata than parted, when erasing and recreating partitions.

also, i'd avoid the constant useless warnings and questions output by parted even when running in script mode, when it should just shut up.

BTW, i wish these silly tools would just let you edit the extent (start/end) of partitions without having to delete and recreate them, but hell...