Use the correct source IP when binding multiple IPs

When the server is listening on multiple interfaces or interfaces with
multiple IPs, the outgoing datagrams are sometime delivered with the
wrong source IP address.

In order to fix that, each quic connection needs to extract the
destination IP (and optionally interface id) of the received datagrams,
and set it as source IP (and interface) on the sent datagrams.

On most platforms, this can be done using ancillary data with recvmsg()
and sendmsg(). Some of the machinery for this is already there for ECN,
this change extends it to read the destination IP info and write it to
the outgoing packets.

Fix #1736

conn_helper_linux.go

@@ -5,3 +5,13 @@ package quic
 import "golang.org/x/sys/unix"
 
 const msgTypeIPTOS = unix.IP_TOS
+
+const (
+	ipv4RECVPKTINFO = 0x8
+	ipv6RECVPKTINFO = 0x31
+)
+
+const (
+	msgTypeIPv4PKTINFO = 0x8
+	msgTypeIPv6PKTINFO = 0x32
+)