[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/wallet/feebumper.cpp

@@ -14,7 +14,9 @@
 #include <validation.h> //for mempool access
 #include <txmempool.h>
 #include <util/moneystr.h>
+#include <util/rbf.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <net.h>
 
 //! Check whether transaction has descendant in wallet or mempool, or has been