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
Add disklabel64 support

MFC after:	2 weeks