[B.A.T.M.A.N.] Adding of basic gw_node/batman_if refcnt

[Sven Eckelmann]

Hi,

if anyone wants to start to track all usage cases of the batman_if and gw_node
then he can use the functions defined in following two patches. The patches are
not fixes, but they provide a starting point for the actual bugfix.

Best regards,
        Sven