Option with non-eager optional argument, like --backup[=CONTROL]

In GNU `getopt_long`, if an option has an optional argument, then it has to be in the same element of `char *argv[]`, connected with an equals sign. Thus, `numbered` is parsed as the argument of the option in `cp --backup=numbered source directory/` but as a separate argument (and treated as a file name) in `cp --backup numbered source directory/`. AFAICT, System.CommandLine cannot be configured to behave this way.