[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`

1 files changed, 17 insertions, 0 deletions

diff --git a/src/util/rbf.cpp b/src/util/rbf.cpp
new file mode 100644
index 0000000000..d520a9606d
--- /dev/null
+++ b/src/util/rbf.cpp
@@ -0,0 +1,17 @@
+// Copyright (c) 2016-2018 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/rbf.h>
+
+#include <primitives/transaction.h>
+
+bool SignalsOptInRBF(const CTransaction &tx)
+{
+    for (const CTxIn &txin : tx.vin) {
+        if (txin.nSequence <= MAX_BIP125_RBF_SEQUENCE) {
+            return true;
+        }
+    }
+    return false;
+}