Fix split mode for all protocols but DoH

In split mode, the code must check for ipv6 availability to return the
correct network stack. Otherwise, we may end up using "tcp6-tls" even
though the upstream IP is an ipv4.

net.go

@@ -0,0 +1,46 @@
+package ctrld
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"tailscale.com/logtail/backoff"
+
+	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
+)
+
+var (
+	hasIPv6Once   sync.Once
+	ipv6Available atomic.Bool
+)
+
+func hasIPv6() bool {
+	hasIPv6Once.Do(func() {
+		ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+		defer cancel()
+		val := ctrldnet.IPv6Available(ctx)
+		ipv6Available.Store(val)
+		go probingIPv6(val)
+	})
+	return ipv6Available.Load()
+}
+
+// TODO(cuonglm): doing poll check natively for supported platforms.
+func probingIPv6(old bool) {
+	b := backoff.NewBackoff("probingIPv6", func(format string, args ...any) {}, 30*time.Second)
+	bCtx := context.Background()
+	for {
+		func() {
+			ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+			defer cancel()
+			cur := ctrldnet.IPv6Available(ctx)
+			if ipv6Available.CompareAndSwap(old, cur) {
+				old = cur
+			}
+		}()
+		b.BackOff(bCtx, errors.New("no change"))
+	}
+}