Changes

ggsn: use libosmocore tundev API to create apn tun device

This way we can start dropping old osmo-ggsn specific API, avoiding
duplication of code.
Moreover, the osmo-ggsn code is using older ioctl APIs, which are
discouraged nowadays in favour of netlink, which osmo_tundev/osmo_netdev
from libosmocore is used.

While doing this, BSD code is dropped since anyway it's not been
maintained for a long time.
If needed, the BSD support can be added to libosmocore
osmo_tundev/osmo_netdev API.

This is a first step (already working). Follow-up commits will replace
the APIs to set up routes and addresses, and later on osmo-ggsn will win
support to set MTU on the interface.

Furthermore, this will allow easily adding netns support to osmo-ggsn
later on if ever needed.

Change-Id: I4d99ba147ac0f3b414d2efef0068b6b8d6cf0014