[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/qt/signverifymessagedialog.cpp

@@ -11,7 +11,7 @@
 #include <qt/walletmodel.h>
 
 #include <key_io.h>
-#include <validation.h> // For strMessageMagic
+#include <util/validation.h> // For strMessageMagic
 #include <wallet/wallet.h>
 
 #include <string>