[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/url.cpp

@@ -0,0 +1,21 @@
+// Copyright (c) 2015-2018 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/url.h>
+
+#include <event2/http.h>
+#include <stdlib.h>
+#include <string>
+
+std::string urlDecode(const std::string &urlEncoded) {
+    std::string res;
+    if (!urlEncoded.empty()) {
+        char *decoded = evhttp_uridecode(urlEncoded.c_str(), false, nullptr);
+        if (decoded) {
+            res = std::string(decoded);
+            free(decoded);
+        }
+    }
+    return res;
+}