fix AVPF management in offer-answer. Initiate an implicit avpf offer while implicit avpf is locally disabled
should result in avpf disabled locally for these payloads.

implicit avpf bugfixes: do not configure encoders to use avpf if avpf is not accepted or offered by remote.

- ZRTP engine is always initialised even when not selected

-AVP implicit to AVP implicit calls now using RTCP feedback packets.
-Fixing bugs in call_tester.c

Fixinf error for feature implicit AVPF in sal_sdp.c and linphonecall.c

Added boolean parameter implicit_rtcp_fb in LinphoneCallParams and SalStreamDescription  to enable or disable the forcing of AVPF messages feature in AVP.
By default, the feature is enabled and RTCP feedback messages PLI, SLI, RPSI and FIR are supported.

Most problematic was that declined streams were not put in SDP answers.

Do not use anymore nb_streams but SAL_MEDIA_DESCRIPTION_MAX_STREAMS and skip inactive ones because active streams won't be at the start of the array anymore

Negotiate RPSI as positive feedback that is the way we are using it in Linphone, keeping compatibility with older versions of Linphone.

Some attributes could be added by us on incoming call whereas it should
not be done.