[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/validation.h

@@ -150,7 +150,6 @@ extern CTxMemPool mempool;
 extern std::atomic_bool g_is_mempool_loaded;
 typedef std::unordered_map<uint256, CBlockIndex*, BlockHasher> BlockMap;
 extern BlockMap& mapBlockIndex GUARDED_BY(cs_main);
-extern const std::string strMessageMagic;
 extern Mutex g_best_block_mutex;
 extern std::condition_variable g_best_block_cv;
 extern uint256 g_best_block;
@@ -298,9 +297,6 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
                         bool* pfMissingInputs, std::list<CTransactionRef>* plTxnReplaced,
                         bool bypass_limits, const CAmount nAbsurdFee, bool test_accept=false) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-/** Convert CValidationState to a human-readable message for logging */
-std::string FormatStateMessage(const CValidationState &state);
-
 /** Get the BIP9 state for a given deployment at the current tip. */
 ThresholdState VersionBitsTipState(const Consensus::Params& params, Consensus::DeploymentPos pos);