Peter had repocopied sys/disklabel.h to sys/diskpc98.h and sys/diskmbr.h.
These two new copies are still intact copies of disklabel.h and
therefore protected by #ifndef _SYS_DISKLABEL_H_ so #including them
in programs which already include <sys.disklabel.h> is currently a
no-op.
This commit adds a number of such #includes.
Once I have verified that I have fixed all the places which need fixing,
I will commit the updated versions of the three #include files.
Sponsored by: DARPA & NAI Labs.
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