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.

qa/pull-tester/rpc-tests.sh

@@ -11,6 +11,7 @@ export BITCOIND=${REAL_BITCOIND}
 #Run the tests
 
 testScripts=(
+    'wallet_protectcoinbase.py'
     'wallet.py'
     'listtransactions.py'
     'mempool_resurrect_test.py'