batman-adv: make the GW selection class algorithm specific

The B.A.T.M.A.N. V algorithm uses a different metric compared to its
predecessor and for this reason the logic used to compute the best
Gateway is also changed. This means that the GW selection class
fed to this logic has a semantics that depends on the algorithm being
used.

Make the parsing and printing routine of the GW selection class
routing algorithm specific. Each algorithm can now parse (and print)
this value independently.

If no API is provided by any algorithm, the default is to use the
current mechanism of considering such value like an integer between
1 and 255.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>