[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/mining.cpp

@@ -21,8 +21,10 @@
 #include <rpc/util.h>
 #include <shutdown.h>
 #include <txmempool.h>
+#include <util/fees.h>
 #include <util/strencodings.h>
 #include <util/system.h>
+#include <util/validation.h>
 #include <validation.h>
 #include <validationinterface.h>
 #include <versionbitsinfo.h>