Changes

ggsn: Use osmo_netdev_addaddr() libosmocore API

This allows dropping a lot of duplicated code.
While at it, drop references to non-linux systems, which are not
maintained anymore.
Param "dstaddr" was not used anywhere in the old APIs, so it can also be
dropped.
Param "tun->routes" was also used in BSD code, so it can also be
dropped.

Change-Id: I1fccfd658542481cd61536fbd3c7a3a32a1c253b