[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/util/validation.cpp

@@ -0,0 +1,20 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <util/validation.h>
+
+#include <consensus/validation.h>
+#include <tinyformat.h>
+
+/** Convert CValidationState to a human-readable message for logging */
+std::string FormatStateMessage(const CValidationState &state)
+{
+    return strprintf("%s%s (code %i)",
+        state.GetRejectReason(),
+        state.GetDebugMessage().empty() ? "" : ", "+state.GetDebugMessage(),
+        state.GetRejectCode());
+}
+
+const std::string strMessageMagic = "Bitcoin Signed Message:\n";