Adopt a terser specification of an MBR partition table of
64 zeros, suggested by miod@ a while ago.

ok mlarkin@

drop detection code for 386sx/386dx CPUs

OpenBSD/i386 doesn't actually support running on 386sx or 386dx CPUs
so we don't need to test whether we're running on one of these CPUs
anymore.

The 486 (w

drop detection code for 386sx/386dx CPUs

OpenBSD/i386 doesn't actually support running on 386sx or 386dx CPUs
so we don't need to test whether we're running on one of these CPUs
anymore.

The 486 (which was launched in 1989) added a few new features over
the 386:
- an alignment check flag in EFLAGS
- 3 new userland instructions: bswap / cmpxchg / xadd
- 3 new kernel mode instructions: invd / wbinvd / invlpg
- new bits in CR0 (386 CPUs did not support ring0 write protection)
- new bits in CR3

In this diff, we remove the code that checks for the alignment check
flag as we've only supported Pentium (or newer) CPUs for some time.

The rest of the diff is about updating comments related to pre-486
CPUs. 2 files under arch/amd64 are updated to keep them in sync
with the arch/i386 updates.

ok mlarkin@, jsg@; tweak from miod@

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Delete the ancient "hold Shift key to force CHS" code. Noone has needed
this in decades.
ok sthen krw

fdisk(8) no longer uses the partition table it finds in
/usr/mdec/mbr. Make the partition table all zeros and
see if any hidden uses fall out.

ok deraadt@

Fix serial port output.

OK weingart@

Remove unused define CHAR_CHS_FORCE.

OK deraadt@

"of of" is wrong - it should be "off of" - but "from" is even nicer

Bring in sync with i386 and enable interrupts before halt when we error
out. Allows use of Ctl-Alt-Del to reboot in case of failure.

ok weingart@, deraadt@

das boot; das cloned das from das i386