[plugin] mark more interfaces private

Numerous Plugin methods are not expected to be called from outside
the base class. Mark them private with a leading underscore:

  _copy_dir()
  _copy_node()
  _copy_symlink()
  _do_copy_path()

  _expand_copy_spec()
  _mangle_command()
  _make_command_filename()
  _is_forbidden_path()

  _collect_copy_specs()
  _collect_cmd_output()
  _collect_strings()

A couple of these (_mangle_command() and _do_copy_path()) are
invoked from the Plugin test suite; update the relevant cases to
call the new names.

Fixes #348.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>