Show @filename in options list

[remkop]

End users may not be aware that they can provide options and positional parameters in @-argument files. If this functionality is enabled, it may make sense to show this in the usage help message in the options list.

For example, javac:

  ...
  -J<flag>                   Pass <flag> directly to the runtime system
  -Werror                    Terminate compilation if warnings occur
  @<filename>                Read options and filenames from file

Javadoc shows this in the synopsis:

Usage: javadoc [options] [packagenames] [sourcefiles] [@files]

In java 10, the java executable:

    @argument files
                  one or more argument files containing options
    -disable-@files
                  prevent further argument file expansion