[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/rpc/rawtransaction_util.cpp

@@ -11,12 +11,12 @@
 #include <key_io.h>
 #include <keystore.h>
 #include <policy/policy.h>
-#include <policy/rbf.h>
 #include <primitives/transaction.h>
 #include <rpc/protocol.h>
 #include <rpc/util.h>
 #include <tinyformat.h>
 #include <univalue.h>
+#include <util/rbf.h>
 #include <util/strencodings.h>
 
 CMutableTransaction ConstructTransaction(const UniValue& inputs_in, const UniValue& outputs_in, const UniValue& locktime, const UniValue& rbf)