I made a mistake in commit 13a64cfe
("Simplify definition of packets by computing automatically their codes").
My intention was that the code an answer packet continues to only differ by the
highest bit, as implemented now by this commit.

Before:
  0x0001, 0x8002   Ask1, Answer1
  0x0003           Notify2
  0x0004, 0x8005   Ask3, Answer3
  0x0006, 0x8007   Ask4, Answer4

After:
  0x0001, 0x8001   Ask1, Answer1
  0x0002           Notify2
  0x0003, 0x8003   Ask3, Answer3
  0x0004, 0x8004   Ask4, Answer4

This makes the protocol easier to document.

And by not wasting the range of possible values, it seems we have enough
space to shrink to a single byte.

This also removes code that became meaningless since that codes are generated
automatically.