controlclient, magicsock: add debug knob to request IPv6 endpoints

Add opt-in method to request IPv6 endpoints from the control plane.
For now they should just be skipped. A previous version of this CL was
unconditional and reportedly had problems that I can't reproduce. So
make it a knob until the mystery is solved.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/direct.go

@@ -18,6 +18,7 @@ import (
 	"net/http"
 	"os"
 	"runtime"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -447,13 +448,18 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 	allowStream := maxPolls != 1
 	c.logf("PollNetMap: stream=%v :%v %v\n", maxPolls, localPort, ep)
 
+	// TODO(bradfitz): once this is verified to not be problematic, remove this
+	// knob and hard-code true below.
+	includeIPv6, _ := strconv.ParseBool(os.Getenv("DEBUG_INCLUDE_IPV6"))
+
 	request := tailcfg.MapRequest{
-		Version:   4,
-		KeepAlive: c.keepAlive,
-		NodeKey:   tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
-		Endpoints: ep,
-		Stream:    allowStream,
-		Hostinfo:  hostinfo,
+		Version:     4,
+		IncludeIPv6: includeIPv6,
+		KeepAlive:   c.keepAlive,
+		NodeKey:     tailcfg.NodeKey(persist.PrivateNodeKey.Public()),
+		Endpoints:   ep,
+		Stream:      allowStream,
+		Hostinfo:    hostinfo,
 	}
 	if c.newDecompressor != nil {
 		request.Compress = "zstd"