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.

doc/release-notes.md

@@ -63,6 +63,16 @@ RPC changes
 
 - The `createrawtransaction` RPC will now accept an array or dictionary (kept for compatibility) for the `outputs` parameter. This means the order of transaction outputs can be specified by the client.
 - The `fundrawtransaction` RPC will reject the previously deprecated `reserveChangeKey` option.
+- Wallet `getnewaddress` and `addmultisigaddress` RPC `account` named
+  parameters have been renamed to `label` with no change in behavior.
+- Wallet `getlabeladdress`, `getreceivedbylabel`, `listreceivedbylabel`, and
+  `setlabel` RPCs have been added to replace `getaccountaddress`,
+  `getreceivedbyaccount`, `listreceivedbyaccount`, and `setaccount` RPCs,
+  which are now deprecated. There is no change in behavior between the
+  new RPCs and deprecated RPCs.
+- Wallet `listreceivedbylabel`, `listreceivedbyaccount` and `listunspent` RPCs
+  add `label` fields to returned JSON objects that previously only had
+  `account` fields.
 
 External wallet files
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