Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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@

Correct typo in usage message.

Merged from boot0cfg.
- Update the mkrdev function.
- Attempt gctl verb "write MBR" when updating.

Reduce diffs from boot0cfg.c (no functional change).

Use ascii null char ('\0') instead of NULL.

Submitted by: Stefan Farfeleder <stefan@fafoe.narf.at>

s/DIOCGPC98/DIOCSPC98/

Use DIOCGPC98 ioctl to write boot block.

Constify and set WARNS=2.

Submitted by: Mike Barcroft <mike@q9media.com>
Reviewed by: md5(1)

Added PC-98 boot manager installation and configuration utility.