Once: start something else on the next start, and only that one

A program that owns the whole machine had nowhere to run. It cannot be
started from the shell, because starting it means there is no shell, and
pointing boot.cfg at it means a machine that keeps starting it - which is a
poor place to find a mistake in something written five minutes ago.

Once writes /System/Boot/once.cfg, in the same format as boot.cfg and read
with the same routines, because a second format for one setting would be a
second format. The loader reads it before boot.cfg and DELETES IT BEFORE IT
JUMPS, which is the only moment there is: after the jump the loader does
not exist.

Consumed by being read rather than by working, so a one shot that hangs
cannot hang twice - the request is gone before the image ran, and the next
start reads boot.cfg like any other.

THE BOOT STATE IS NOT TOUCHED, and the first version got that wrong. It
marked the start the way any other start is marked, and then every
successful bare metal boot reported that it had never arrived - because a
program with the whole machine has no filesystem to clear a mark with and
is doing nothing wrong by not having one. Found by running it: the image
printed its line and the next start still said the last one did not.

Three disks, each a start further along, so none of the tests depends on
another having run.

The loop is closed on the machine now: write it in Edit, assemble it with
Asm, ask for it with Once, restart, watch it own the machine, and the
system comes back without being asked.

Tests/expected/onceDue.out

@@ -0,0 +1,5 @@
+stage two
+just this once: /System/Boot/bare.bin
+bare metal: no system, just this
+Execution halted.
+[exit 0]