doc: Add Handsfree Audio Card "Connect" errors
bluetooth: Rename DUN GW plugin

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.
dun_gw: Add BlueZ 5 version

This patch adds initial implementation of the D-Bus Profile1
interface and methods responsible for handling Bluetooth connections.

The DUN GW profile interface is registered as soon as a GPRS capable
modem is registered in oFono.
dun_gw_bluez5: Fix copy-paste error
dun_gw_bluez5: Fix sanity check

We should be checking that modems != NULL, not modems->data.  This is
because the list can be empty and we would crash.
hfp_ag_bluez5: Fix sanity check

We should be checking that modems != NULL, not modems->data.  This is
because the list can be empty and we would crash.
handsfree-audio: Add support for initiating SCO connections

When calling the card's .Connect() method, we should be able to
establish a SCO connection.

Right now, we only have support for establishing the SCO connection
directly, this is what is expected from HFP 1.5 HF/AG devices.
handsfree-audio: Add support for sending the SCO socket

Send the SCO socket to the agent associated with the card that
just got connected.
handsfree-audio: Reject .Connect() from other senders

Only the agent should be able to call .Connect() on the card.
hfp_hf_bluez5: Add a card driver for HFP 1.6

This is just the skeleton of a Handsfree Audio Card for the HF side of
HFP 1.6.
hfp_ag: Add Handsfree Audio Card to emulator
hfp_ag: Create/remove Audio Card in emulator
hfp_ag: Set card's local and remote address
hfp_ag: Register card when SLC is established
hfp_ag: Remove card when disconnect HFP AG profile
emulator: Change feature check to attend HFP 1.6
Revert "handsfree-audio: Reject .Connect() from other senders"

This reverts commit 32d18976879bbc93da811ab88eade4e98bb248fa.