ddns-scripts: Added support for custom update scripts

Squashed commit of the following:

commit 2701c8868e1ef4949db57e53b27958edba6abb59
Author: Christian Schoenebeck <christian.schoenebeck@gmail.com>
Date:   Sun Oct 5 11:01:57 2014 +0200

    ddns-scripts: Added support for custom update scripts

    Sample script
    Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>

commit e07ecb90fa2c7404a97cf64024e89bd9d88aacd4
Author: Christian Schoenebeck <christian.schoenebeck@gmail.com>
Date:   Sun Oct 5 11:00:11 2014 +0200

    ddns-scripts: Added support for custom update scripts

    Added support for custom update scripts with new option update_script.
    function get_service_url() renamed to get_service_data() and extended to detect scripts inside service / service_ipv6 for later use
    function send_update() modified to support custom update scripts.
    Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>

commit 39e41b2151a79a7ace71a9d40b87cd4d6ce09503
Author: Christian Schoenebeck <christian.schoenebeck@gmail.com>
Date:   Sun Oct 5 10:52:44 2014 +0200

    ddns-scripts: Added support for custom update scripts

    Added support for custom update scripts with new option update_script
    Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>

commit 33f264768e37d8a6fe564faaafa51a7b45a0ee19
Author: Christian Schoenebeck <christian.schoenebeck@gmail.com>
Date:   Sun Oct 5 10:48:21 2014 +0200

    ddns-scripts: Insert description for NEW option update_script

    Insert description for NEW option update_script
    Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>

commit 6f6a60244df53e0556f5c75845c69aa832f97e4c
Author: Christian Schoenebeck <christian.schoenebeck@gmail.com>
Date:   Sun Oct 5 10:43:52 2014 +0200

    ddns-scripts: Update PKG_RELEASE

    Update_PKG_RELEASE to reflect changes
    Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>