xsd-fu: Refactor command-line option handling

- Add support for long options
- Allow individual package namespaces to be specified
- Language classes maintain the default and current namespace in
  use for each package
- The dry run option is now functional
- ant and maven builds use long options; maven no longer specifies
  package namespaces explicitly; ant uses them to ensure they are
  functional, but this is not required since the defaults are fine
- Unify usage messages
- Add options and subcommands to usage message
- Update documentation to include options, subcommands and usage changes