bluetooth: add correct HFP rfcomm negotiation

HFP 1.6 requires a stateful negotiation of AT commands.  The prior
version got away with initialising HFP simply by replying 'OK' to
every negotiation attempt.  This one actually tries to parse the state
and make sure the negotiation occurs correctly

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

---

v4:

- Update for PA 11.0
- Finally sort out CIND negotiaton for complex headsets

v3:

- remove internal debugging
- added comment for t->config being not null for hfp
- removed unused returns from hfp_rfcomm_handle()
- remove rfcomm comment
- use pa_startswith
- simplify negotiation

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/491>

src/modules/bluetooth/bluez5-util.c

@@ -172,7 +172,10 @@ pa_bluetooth_transport *pa_bluetooth_transport_new(pa_bluetooth_device *d, const
 
     if (size > 0) {
         t->config = pa_xnew(uint8_t, size);
-        memcpy(t->config, config, size);
+        if (config)
+            memcpy(t->config, config, size);
+        else
+            memset(t->config, 0, size);
     }
 
     return t;