Change <sys/diskpc98.h> to not redefine the same symbols that are
being defined in <sys/diskmbr.h>. Instead give the symbols here a
"PC98_" prefix. This way, both <sys/diskmbr.h> and <sys/diskpc98.h>
can be included in the same C source file.

The renaming is trivial. The only gotcha is that DOSBBSECTOR is
also redefined from 0 to 1. This because DOSBBSECTOR was always
used in conjunction with an addition of 1. The PC98_BBSECTOR symbol
is defined as 1 and the expression is simplified.

Note: it is not believed that ports are seriously impacted; or at
all for that matter.

Approved by: nyan@
Fill in the description fields for M_FICT_PAGES.

Reviewed by:	kib
MFC after:	3 days