[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.cpp

@@ -69,13 +69,3 @@ void InitWarning(const std::string& str)
 {
     uiInterface.ThreadSafeMessageBox(str, "", CClientUIInterface::MSG_WARNING);
 }
-
-std::string AmountHighWarn(const std::string& optname)
-{
-    return strprintf(_("%s is set very high!"), optname);
-}
-
-std::string AmountErrMsg(const char* const optname, const std::string& strValue)
-{
-    return strprintf(_("Invalid amount for -%s=<amount>: '%s'"), optname, strValue);
-}