HouseFly 2.19.0 — it sees the lamp it lands on

On the sandboxes, a week after 2.16.0, each fly was "refused" about six
hundred times a day and acted about twice. None of those were the safety layer
protecting anything. Three things, all fixed:

• The deadband compared a wish with the fly's own last command, so it never
  noticed anyone else changing a light. It now compares with the light as it
  actually is: a fly that lands on a lamp another fly or a person has switched
  off sees it off.
• A wish that matched the lamp was counted as a safety refusal. It is now
  counted apart, as total_already_as_wanted; total_blocked means the
  safety layer said no.
• The fly's state barely moved what it asked for. At gains of 0.5 on valence
  and 0.2 on arousal every wish sat between about 0.45 and 0.6 — inside the
  0.15 deadband — so nothing it learned or felt could reach a light. At 2.0
  and 0.4, a learned valence shift of 0.1 is a change of 0.2, and its day is
  bright when it is lively and dim when it is drowsy.

A switch is now asked for as on or off rather than as a level, so "already as
it wants it" means the same thing for a switch as for a lamp. Six new checks
in section 10b of tools/validate.py.