Refactor readdoslabel() into a more readable form using various

Refactor readdoslabel() into a more readable form using various
helper functions.

The refactored code ensures disklabels are read from/written to
disk only from/to unused space or an OpenBSD partition. This
prevents accidental damage to filesystems that start immediately
following an MBR or GPT.

The refactored code also finds the disklabel present on the
i386/amd64 floppyXX.img, rather than spoofing the media as a
single MSDOS partition.

Tweak and positive comments from jmatthew@