batman-adv: add seqno maximum age and protection start flag parameters

To allow future use of the window protected function with different
maximum sequence numbers, add a parameter to set this value which
was previously hardcoded. Another parameter added for future use is a
flag to return whether the protection window has started.

While at it, also fix the kerneldoc.

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>