value as reserved for internal use in boot blocks, because RB_PAUSE
broke binary compatibility by usurping the RB_DUAL flag. Probably no
one except me has boot blocks for which this matters, since most boot
blocks based on biosboot including pc98's boot2 can't boot elf kernels,
and /boot/loader doesn't properly pass flags set by the previous stage.
reboot.h:
Also mark the historical RB_PROBEKBD flag (0x80000) as reserved for
internal use in boot blocks.
boot2.c:
Added comments to inhibit usurping of other flags.
Approved by: guido, imp
MFC after: 1 week
4d450ff944