Better argument parsing

Use a C++ command line argument library for defining and parsing ROOT’s options.

ROOT’s binaries understand a mixture of short and long arguments. The parsing is done in C++, with ad hoc C++ code. We would like to switch to a C++ command line argument library. This would imply implementing ROOT’s options (e.g. in TApplication.cxx and rootcling_impl.cxx) using that library.

This is a companion discussion topic for the original entry at