Achieve this by adding a MAIN_FUNCTION macro, consolidating the docs, and
introducing the macro across our distributed binaries.
Also update the docs to explain that anyone using binutils < 2.36 is
effected by this issue. Release builds are not, because they use binutils
2.37. Currently LTS Linux distros, like Ubuntu Focal, ship with 2.34.
https://packages.ubuntu.com/focal/binutils
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.
