Return improved error message when trying to spend Coinbase coins (#1373).

Extra parameter added to AvailableCoins to include or exclude Coinbase coins.
SelectCoins, used for sending taddr->taddr, will exclude Coinbase coins.

Added qa rpc test and a runtime parameter -regtestprotectcoinbase to enforce
the coinbase->zaddr consensus rule in regtest mode.

src/chainparams.h

@@ -83,6 +83,8 @@ public:
     std::string GetFoundersRewardAddressAtIndex(int i) const;
     /** #1398 to return a fixed founders reward script for miner_tests */
     bool fMinerTestModeForFoundersRewardScript = false;
+    /** Enforce coinbase consensus rule in regtest mode */
+    void SetRegTestCoinbaseMustBeProtected() { consensus.fCoinbaseMustBeProtected = true; }
 protected:
     CChainParams() {}