blindfold is removed when control of a side is received

in blindfold_ii, blindfold was taken off when local side takes control
in this fixup, blindfold is taken off as soon as host gives control of
any side.
to achieve this, playturn.cpp must be able to ask the playcontroller to
remove the blindfold.
we create a virtual method of play_controller, on_not_observer,
which is called whenever this client gets local control of a side
(it becomes ai or human)
on all play_controllers except play_mpcontroller, this does nothing
in play_mpcontroller it calls a "remove_blindfold" function
when a human side takes control, remove_blindfold is still called,
as a precaution.