[build] Add several util units

Adds the following util units and adds them to libbitcoin_util:

- `util/url.cpp` takes `urlDecode` from `httpserver.cpp`
- `util/error.cpp` takes `TransactionErrorString` from
  `node/transaction.cpp` and `AmountHighWarn` and `AmountErrMsg` from
  `ui_interface.cpp`
- `util/fees.cpp` takes `StringForFeeReason` and `FeeModeFromString` from `policy/fees.cpp`
- `util/rbf.cpp` takes `SignalsOptInRBF` from `policy/rbf.cpp`
- 'util/validation.cpp` takes `FormatStateMessage` and `strMessageMagic` from 'validation.cpp`

src/ui_interface.h

@@ -129,10 +129,6 @@ void InitWarning(const std::string& str);
 /** Show error message **/
 bool InitError(const std::string& str);
 
-std::string AmountHighWarn(const std::string& optname);
-
-std::string AmountErrMsg(const char* const optname, const std::string& strValue);
-
 extern CClientUIInterface uiInterface;
 
 #endif // BITCOIN_UI_INTERFACE_H