Argh! Fix a brainfart of mine. In the old boot0, we relocated ourself

to 0x600 via a 'rep movsw'.  Once that was done, %cx was zero, so we could
simply use 'movb' to update the lower byte of %cx in preparation for
zeroing out the fake partition entry used to boot to other drives via F5.
Well, in the new boot0, we don't actually relocate ourselves, instead it
is easier to create the fake partition entry first and then just use it to
get the BIOS to load all of boot0 into memory at 0x600.  However, since we
aren't doing the relocate code anymore, we don't know that %cx == 0 when
we hit the 'movb' to setup %cx for clearning the fake partition entry.
Thus, if %ch != 0 when the BIOS started boot0, then it would end up zeroing
a lot more memory than just 8 words.  The solution is to do a word move of
$8 into %cx.

Debugging help from:	David Wolfskill <dhw@whistle.com>

sys/boot/i386/boot0/boot0.s

@@ -81,7 +81,7 @@ start:		cld				# String ops inc
 #
 		movw $fake,%bp			# Address variables
 		movw %bp,%di			# %di used in stosw
-		movb $0x8,%cl			# Words to clear
+		movw $0x8,%cx			# Words to clear
 		rep				# Zero
 		stosw				#  them
 		incb -0xe(%di)			# Sector number 1