Reboot, and the machine device that makes it possible

Until now the only way to restart was to stop the emulator and run it
again, which meant the one thing the machine could not do was the thing
Once was written for. The loop now closes without leaving it:

  > Once /System/Boot/bare.bin
  next start: /System/Boot/bare.bin, once
  > Reboot
  starting again
  stage two
  just this once: /System/Boot/bare.bin
  bare metal: no system, just this

Writing 1 to port 0x13 asks the machine to start over. A PORT RATHER THAN A
SERVICE, because a reset has to work when the system does not: something
only askable through SWI would be unavailable in exactly the case that
wants it most, and a program that owns the whole machine has no system to
ask. It is device class 0x04, in the range kept for the machine rather than
among the peripherals, because it is not one - it is not attached to
anything and cannot be unplugged.

WHAT A RESET REPEATS IS HOW THE MACHINE STARTED. Named an image, the
emulator places it again; named none, the ROM is shadowed again and reads
the disk. Anything else would mean a reset changed what the machine IS,
which is the one thing a reset must not do. Both are tested.

Taken between instructions, because a device cannot restart the machine
from inside the instruction that asked: the CPU is part way through a step
and its state is not yet anything a reset could leave behind consistently.

The disk stays attached and keeps everything written to it - that is what
warm means. The vector table is cleared, which is the one deliberate
departure from leaving memory alone: a vector points into whatever
installed it, and after a reset that program is not running, so a handler
left behind would aim an interrupt at an address belonging to something
gone. It is the argument CosmOS already makes at exit, applied to the
machine.

Reboot is 45 bytes, most of them the word it prints.

Tests/expected/rebootDirect.out

@@ -0,0 +1,6 @@
+CosmOS
+> starting again
+CosmOS
+> halted
+Execution halted.
+[exit 0]

Tests/expected/rebootOnce.out

@@ -0,0 +1,10 @@
+stage two
+CosmOS
+> next start: /System/Boot/bare.bin, once
+finished
+> starting again
+stage two
+just this once: /System/Boot/bare.bin
+bare metal: no system, just this
+Execution halted.
+[exit 0]