Rename account to label where appropriate

This change only updates strings and adds RPC aliases, but should simplify the
implementation of address labels in
https://github.com/bitcoin/bitcoin/pull/7729, by getting renaming out of the
way and letting it focus on semantics.

The difference between accounts and labels is that labels apply only to
addresses, while accounts apply to both addresses and transactions
(transactions have "from" and "to" accounts). The code associating accounts
with transactions is clumsy and unreliable so we would like get rid of it.

src/rpc/protocol.h

@@ -76,7 +76,7 @@ enum RPCErrorCode
     //! Wallet errors
     RPC_WALLET_ERROR                = -4,  //!< Unspecified problem with wallet (key not found etc.)
     RPC_WALLET_INSUFFICIENT_FUNDS   = -6,  //!< Not enough funds in wallet or account
-    RPC_WALLET_INVALID_ACCOUNT_NAME = -11, //!< Invalid account name
+    RPC_WALLET_INVALID_LABEL_NAME   = -11, //!< Invalid label name
     RPC_WALLET_KEYPOOL_RAN_OUT      = -12, //!< Keypool ran out, call keypoolrefill first
     RPC_WALLET_UNLOCK_NEEDED        = -13, //!< Enter the wallet passphrase with walletpassphrase first
     RPC_WALLET_PASSPHRASE_INCORRECT = -14, //!< The wallet passphrase entered was incorrect
@@ -85,6 +85,9 @@ enum RPCErrorCode
     RPC_WALLET_ALREADY_UNLOCKED     = -17, //!< Wallet is already unlocked
     RPC_WALLET_NOT_FOUND            = -18, //!< Invalid wallet specified
     RPC_WALLET_NOT_SPECIFIED        = -19, //!< No wallet specified (error when there are multiple wallets loaded)
+
+    //! Backwards compatible aliases
+    RPC_WALLET_INVALID_ACCOUNT_NAME = RPC_WALLET_INVALID_LABEL_NAME,
 };
 
 UniValue JSONRPCRequestObj(const std::string& strMethod, const UniValue& params, const UniValue& id);