B1: a boot area on the disk, reserved by arithmetic that was already there

The first rung of booting from disk. A boot area is blocks between the
superblock and the directory that the filesystem never allocates and never
sees, and NOTHING WAS ADDED TO RESERVE THEM: both implementations work out
the first usable block as directoryStart + directoryBlocks, and
directoryStart has always been a field rather than a constant. Formatting
with the directory moved up reserves everything below it. Neither allocator
changed, on either side.

Two new superblock fields in bytes that were reserved: bootBlocks at 14,
per slot, and bootSlot at 16. A disk made before this has zero in both,
which reads as "no boot area" - true, and the same shape as the version two
parent field, where the value an older disk already held was the right
answer without conversion.

TWO SLOTS, ALWAYS. A boot slot is raw blocks with no entry to rename, so
the write-a-temporary-and-rename ordering that protects every file cannot
protect it, and a machine interrupted while updating its only slot would
not boot at all - the one failure on this disk with no way back. Writing
the slot that is not live and then moving one byte makes that a machine
that boots what it had before.

bootBlocks and directoryStart say the same thing from two sides, so a disk
where they disagree is refused rather than guessed at, as is one naming a
slot that does not exist.

Checked where it matters: the HOST formats a disk with a boot area and the
MACHINE fills it, then the reserved blocks are compared against zero. The
machine's allocator is the one that had no idea any of this was happening,
which is what makes that the check worth having. Six host checks besides,
including both halves of the superblock disagreeing.

Tests/disk.sh

@@ -172,6 +172,26 @@ check   "the largest that fits"           "$TOOL" format huge.img 65535 8191
 printf '\x20\x00' | dd of=lying.img bs=1 seek=10 conv=notrunc status=none
 refuses "nor reading one that claims it"  "$TOOL" list lying.img
 
+# ---- A boot area, and the two halves of the superblock that describe it ----
+#
+# bootBlocks and directoryStart say the same thing from two sides, so a disk where they
+# disagree is one where there is no way to tell which is wrong. Both are refused.
+check   "format with a boot area"         "$TOOL" format boot.img 512 4 32
+check   "and it reads back"               "$TOOL" list boot.img
+refuses "no boot area bigger than a disk" "$TOOL" format small.img 32 2 64
+check   "and none at all is still fine"   "$TOOL" format plain.img 64 2
+
+bootField() { python3 -c "
+import sys
+f = open(sys.argv[1], 'r+b'); f.seek(int(sys.argv[2])); f.write(bytes.fromhex(sys.argv[3]))
+" "$@"; }
+cp boot.img lying.boot.img
+bootField lying.boot.img 14 0010          # Claims 16 blocks a slot, directory says 32.
+refuses "nor a boot area that disagrees"  "$TOOL" list lying.boot.img
+cp boot.img badslot.img
+bootField badslot.img 16 07               # Names slot 7, and there are two.
+refuses "nor a slot that does not exist"  "$TOOL" list badslot.img
+
 echo
 if [ "$FAIL" -eq 0 ]; then
     echo "All $PASS disk tool checks passed."