Add DragonFly's partition number to fdisk(8) and diskmbr.h

This change doesn't make any attempt to add support for these slices to the
relevent GEOM classes. Just register the number in fdisk and the canonical
list of kernel macros (diskmbr.h).

Obtained from: DragonFlyBSD (794d80aa519b394b3174f20776a) (small subset of)