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`
https://github.com/bitcoin/bitcoin/pull/15638/commits/91a25d1e711bfc0617027eee18b9777ff368d6b9
Depends on D5994
This is a partial backport of Core PR15638