As BlueZ 5 introduced backwards incompatible API changes, and we want to
keep support for BlueZ 4 based DUN GW plugin for some time, we need to
separate DUN GW plugin which is based on BlueZ 4 from the one based on
BlueZ 5.
The dun_gw.c plugin is renamed to dun_gw_bluez4. This will make it easy
to add a DUN GW plugin for BlueZ 5.
Frédéric Danis