Clash of old "-s" and new "-s dir" options in javac

The new -s dir used to specify the source output directory (and impl in javac.main.Main) clashes with the older undocumented -s switch used to request source output of generated files (and impl in javac.main.JavaCompiler).

This means that if the documented "-s dir" option is used, when the time comes to generate any class files, JavaCompiler will instead try and generate source code instead.