As BlueZ 5 introduced backwards incompatible API changes, and we want to
keep support for BlueZ 4 based plugins for some time, we need to separate
the plugins that are based on BlueZ 4 from the ones based on BlueZ 5.
The bluetooth.c plugin has now been renamed to bluez4.c and the hfp_hf.c
plugin is renamed to hfp_hf_bluez4. This will make it easy to add a
HFP HF plugin for BlueZ 5.
DUN server is probed when modem state changes to online. It registers
DUN record to Bluetooth adapter and wait for incoming DUN connection.
Based on a patch from Zhenhua Zhang <zhenhua.zhang@intel.com>