Add disklabel64 support to GEOM_PART class.

This partitioning scheme is used in DragonFlyBSD. It is similar to
BSD disklabel, but has the following improvements:

• metadata has own dedicated place and isn't accessible through partitions;
• all offsets are 64-bit;
• supports 16 partitions by default (has reserved place for more);
• has reserved place for backup label (but not yet implemented);
• has UUIDs for partitions and partition types;

No objections from: geom
MFC after: 2 weeks
Relnotes: yes