Send an ICMP6_PACKET_TOO_BIG when we try to send a message that is too big. This change also: 1) Completely disables fallback to streams when we've negotiated messages. 2) Drops the default MTU on the bonnet to 1280. gfe-relnote: gfe-relnote: n/a (QBONE-only change) PiperOrigin-RevId: 278862096 Change-Id: I0ff1fae69c7f0a82d3afcd5f13f7c2626f2e583b