Don't require urlDecode function in wallet code since urlDecode implementation
currently uses libevent. Just call urlDecode indirectly though URL_DECODE
function pointer constant if available.
In bitcoind and bitcoin-qt, URL_DECODE is implemented and used to interpret RPC
wallet requests. In bitcoin-wallet, URL_DECODE is null to avoid depending on
libevent.
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`
