net/dns/resolver: permit larger max responses, signal truncation

This raises the maximum DNS response message size from 512 to 4095. This
should be large enough for almost all situations that do not need TCP.
We still do not recognize EDNS, so we will still forward requests that
claim support for a larger response size than 4095 (that will be solved
later). For now, when a response comes back that is too large to fit in
our receive buffer, we now set the truncation flag in the DNS header,
which is an improvement from before but will prompt attempts to use TCP
which isn't supported yet.

On Windows, WSARecvFrom into a buffer that's too small returns an error
in addition to the data. On other OSes, the extra data is silently
discarded. In this case, we prefer the latter so need to catch the error
on Windows.

Partially addresses #1123

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

net/dns/resolver/neterr_windows.go

@@ -5,6 +5,7 @@
 package resolver
 
 import (
+	"errors"
 	"net"
 	"os"
 
@@ -27,3 +28,16 @@ func networkIsUnreachable(err error) bool {
 	// difference between down and unreachable? Add comments.
 	return false
 }
+
+// packetWasTruncated returns true if err indicates truncation but the RecvFrom
+// that generated err was otherwise successful. On Windows, Go's UDP RecvFrom
+// calls WSARecvFrom which returns the WSAEMSGSIZE error code when the received
+// datagram is larger than the provided buffer. When that happens, both a valid
+// size and an error are returned (as per the partial fix for golang/go#14074).
+// If the WSAEMSGSIZE error is returned, then we ignore the error to get
+// semantics similar to the POSIX operating systems. One caveat is that it
+// appears that the source address is not returned when WSAEMSGSIZE occurs, but
+// we do not currently look at the source address.
+func packetWasTruncated(err error) bool {
+	return errors.Is(err, windows.WSAEMSGSIZE)
+}