daemon: reduce the MTU limit in RTP and DTLS streams
Issue generated from Tuleap's migration script. Originally submitted by: Guillaume Roguez (guillaume)
IPv6 has increased the minimal MTU size from 512 to 1280 bytes (see rfc2460, chapter 5, "Packet Size Issues")
So an IPv6 compatible network must use this value as minimal ethernet payload size (we need to remove IP/UDP/xxx headers size when needed).
This gives us a convenient minimal point to use to transmit our UDP data (RTP and DTLS streams) and ensure that our packets are not going to be fragmented and possibly be dropped (in most case).