UDP channel wasn't failing to send an IPv6 packet if IPv6 is not available. This is now fixed and allows fallback to ipv4.

- supports IPv6 DNS servers
- supports nameserver lists with both IPv4 and IPv6 servers

Also, A resolution following SRV results now takes place in parallel.

fix test "bad request" that was not working at all.

Fix channel retry mechanism by using linked list of addrinfo structures instead of a belle_sip_list_t for DNS A and AAAA results.

* fix many bugs and crashes:
  - rport shall not be set if not present in the request
  - received shall not be set if identical to via's host
  - belle_sip_response_get_return_hop() wasn't working without received parameter
  - belle_sip_response_get_return_hop() wasn't working if no rport and no port (that is default port to be used).