MFC: r208777

• In gpart_bootfile_read() fix an off-by-one error preventing the bootstrap file to be of maximum size.
• Add special handling required for SMI/VTOC8 disklabel partcode, i.e. avoid overwriting the label when writing the bootstrap code to the partition starting at 0 and install it to all partitions when the -i option is omitted just like geom_sunlabel(4) and sunlabel(8) do by default.
• Add missing prototypes.
• Add const where applicable.

Reviewed by: marcel