Fix v3 plugins to support returning values back to OpenVPN.

Fix v3 plugins to support returning values back to OpenVPN.

There's a small implementation issue with the V3 plugin API
implementation.  V3 plugins cannot return data back to OpenVPN using
openvpn_plugin_args_func_return since
openvpn_plugin_args_func_return.return_list is always NULL.

This patch makes the behaviour identical to the V2 API (return_list is
again a proper out parameter).

Signed-off-by: Kenneth Rose <ken+openvpn@surfeasy.com>
Acked-by: David Sommerseth <davids@redhat.com>
Message-Id: CAAUdgR8VZjaO0-thFPVoQkbZ1DNG3rx+gtYYLqyfocp3yns==w@mail.gmail.com
URL: http://article.gmane.org/gmane.network.openvpn.devel/7083
Signed-off-by: David Sommerseth <davids@redhat.com>