Start untangling the disklabel from various bits of code with the goal of

Start untangling the disklabel from various bits of code with the goal of
introducing support for a new 64 bit disklabel.

Remove the D_* flags for disklabel.d_flags.  These sorts of flags just do
not belong in the disk image.

Relabel the partition sub-structure in disktab.h, and remove other ancient
compatibility defines in disklabel.h.