Internally whether a coin is frozen is tri-state:
- forced-True, set by the user
- forced-False, set by the user
- unset/default: is_frozen_coin() can decide whether the coin should be frozen
This patch lets set_frozen_state_of_coins() undo a previous explicit setting of True/False,
by calling it with a value of None.
Note: there is still no way in the GUI to undo an explicit setting of True/False.