Fix bug due to interaction of partition table & string interning (#28)

A user reported that the previous commit (03fca656d70) — which
added a partition table for USB drive booting — was causing
SectorLISP to crash on the input
	(CONS (QUOTE AAA) NIL)

This turned out to be because the SectorLISP code interns new
tokens starting at an area where it sees two null bytes.  The
newly added "partition table" entry for a bootable partition
contains multiple 00's in a row, & was followed by code.  The
code was clobbered.

This commit moves the bootable partition table entry towards
the end of the partition table, which is safe to clobber.

sectorlisp.S

@@ -285,9 +285,9 @@ Eval:	test	%ax,%ax				# Eval(e:ax,a:dx):ax
 	call	Evlis
 	xchg	%ax,%si
 	pop	%ax
-	jmp	2f
+	call	Apply
 
-	.fill	0x1BE - (. - _start), 1, 0xce	# to have this boot from a USB
+	.fill	0x1BE - (. - _start), 1, 0x90	# to have this boot from a USB
 						# drive on a modern PC, make a
 						# degenerate "partition table"
 						# where this sector starts the
@@ -297,33 +297,32 @@ Eval:	test	%ax,%ax				# Eval(e:ax,a:dx):ax
 						# starting sector LBA numbers!
 
 						# * 1st partition entry *
-	.byte	0x80				#  - bootable indicator
-	.byte	0, 1, 0				#  - h₀, s₀ (& c₀ hi bits), c₀
-	.byte	0x7F				#  - OS or filesystem indicator
-	.byte	0xFF, 0xFF, 0xFF		#  - h₉, s₉ (& c₉ hi bits), c₉
-	.long	0				#  - c₀*Cₙ + h₀*Hₙ + s₀*Sₙ
-
-2:	call	Apply
-	.byte	0x3C
-						# * 2nd partition entry *
 	.byte	0x00				#  - bootable indicator
+	.byte	0b11010010			#    reads as add %dl,%dl
 	pop	%dx				# restore A
 	mov	%cx,%si				# si = B
 	xchg	%ax,%di
 	call	Gc
 	mov	%dx,%di				# di = A
-	.byte	0x3C				#
 	.byte	0x00				#  - hi8(c₀*Cₙ + h₀*Hₙ + s₀*Sₙ)
+	.byte	0b11010010			#    reads as add %dl,%dl
 	sub	%si,%cx				# cx = C - B
-	rep movsb
-						# * 3rd partition entry *
+	.byte	0x3C				# cmp $0,%al
+						# * 2nd partition entry *
 	.byte	0x00				#  - bootable indicator
-	.byte	0xC9
+	rep movsb
 	mov	%di,%cx				# cx = A + (C - B)
 	pop	%dx				# restore a
 1:	ret
 
-	.fill	0x1DE + 0x8 - (. - _start), 1, 0xce
+	.fill	0x1CE + 0x8 - (. - _start), 1, 0xce
+	.long	0				#  - c₀*Cₙ + h₀*Hₙ + s₀*Sₙ
+
+	.fill	0x1DE - (. - _start), 1, 0xce	# * 3rd partition entry *
+	.byte	0x80				#  - bootable indicator
+	.byte	0, 1, 0				#  - h₀, s₀ (& c₀ hi bits), c₀
+	.byte	0x7F				#  - OS or filesystem indicator
+	.byte	0xFF, 0xFF, 0xFF		#  - h₉, s₉ (& c₉ hi bits), c₉
 	.long	0				#  - c₀*Cₙ + h₀*Hₙ + s₀*Sₙ
 
 	.fill	0x1EE - (. - _start), 1, 0xce	# * 4th partition entry *