wgengine: flush DNS cache after major link change.

Windows has a public dns.Flush used in router_windows.go.
However that won't work for platforms like Linux, where
we need a different flush mechanism for resolved versus
other implementations.

We're instead adding a FlushCaches method to the dns Manager,
which can be made to work on all platforms as needed.

Fixes https://github.com/tailscale/tailscale/issues/2132

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/dns/flush_windows.go

@@ -9,11 +9,20 @@ import (
 	"os/exec"
 )
 
-// Flush clears the local resolver cache.
-func Flush() error {
+func flushCaches() error {
 	out, err := exec.Command("ipconfig", "/flushdns").CombinedOutput()
 	if err != nil {
 		return fmt.Errorf("%v (output: %s)", err, out)
 	}
 	return nil
 }
+
+// Flush clears the local resolver cache.
+//
+// Only Windows has a public dns.Flush, needed in router_windows.go. Other
+// platforms like Linux need a different flush implementation depending on
+// the DNS manager. There is a FlushCaches method on the manager which
+// can be used on all platforms.
+func Flush() error {
+	return flushCaches()
+}