* refactor: [[maybe_unused]] iff arg _might_ be used
If the arg is never used, comment out its name.
If the arg is _sometimes_ used e.g. with ifdefs, use [[maybe_unused]].
Port libtransmission to C++. This PR doesn't refactor everything to c++.
Its code changes are only what was necessary to compile and link as c++.
See libtransmission/README.md for details on how to submit modernization
patches!
Co-authored-by: Mike Gelfand <mikedld@mikedld.com>