Restrict the outputs when a minting-only key is used

OP_COINSTAKE was renamed to OP_MINT because it now also verifies that
the transaction is purely minting, and not moving the coins nor
destroying them.

So using a minting-only key is now only allowed:

* inside a CoinStake transaction (like before)
* when the input and output scripts are all the same
* and when the total output value is greater than or equal to the total
  input value