Auto merge of #1431 - bitcartel:master_1373_taddr_coinbase_error, r=bitcartel

Return a more informative error message when trying to spend coinbase; select non-coinbase inputs when sending to a transparent output if needed

For #1373 and #1519

Code change:
- Extra parameter added to AvailableCoins to include or exclude Coinbase coins.  Default value of parameter is 'true' as current behaviour is to include Coinbase coins.
- SelectCoins, used for sending taddr->taddr, will now exclude Coinbase coins.

Unit test:
Tried to write a test to focus on the extra parameter added to AvailableCoins but could not.

Empirical testing on Testnet:
Current behaviour is that upstream RPC commands sendfrom and sendtoaddress try to spend coinbase coins returned by AvailableCoins.  So the user will see:

```
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 1000.0
error: {"code":-6,"message":"Insufficient funds"}

./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
error: {"code":-4,"message":"Error: The transaction was rejected! This might happen if some of the coins in your wallet were already spent, such as if you used a copy of wallet.dat and coins were spent in the copy but not marked as spent here."}

./zcash-cli sendfrom "" mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
error: {"code":-4,"message":"Error: The transaction was rejected! This might happen if some of the coins in your wallet were already spent, such as if you used a copy of wallet.dat and coins were spent in the copy but not marked as spent here."}
```

After fix is applied:

```
./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 1000.0
error: {"code":-6,"message":"Insufficient funds"}

./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
error: {"code":-4,"message":"Coinbase funds can only be sent to a zaddr"}
```

When non-coinbase UTXOs exist, they will now be selected and used:

```
./zcash-cli z_sendmany tnPJZHeVxegCg91utaquBRPEDBGjozfz9iLDHt7zvphFbZdspNgkTVLCGjDcadQBKNyUwKs8pNjDXuEZKrE1aNLpFwHgz4t '[{"address":"mx5fTRhLZwbYE7ZqhAPueZgQGSnwTbdvKU", "amount":0.01}]'

./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 1000.0
error: {"code":-6,"message":"Insufficient funds"}

./zcash-cli sendtoaddress mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
9818e543ac2f689d4ce8b52087607d73fecd771d45d316a1d9db092f0485aff2

./zcash-cli sendfrom "" mrEGRmGJhmwAa4MQjzGd86ry63vrvovu9b 0.00003000
899f2894823f51f15fc73b5e0871ac943edbe0ff88e1635f86906087b72caf30
```

qa/pull-tester/rpc-tests.sh

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