envknob: support changing envknobs post-init

Updates #5114

Change-Id: Ia423fc7486e1b3f3180a26308278be0086fae49b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_windows.go

@@ -32,7 +32,7 @@ const (
 	versionKey = `SOFTWARE\Microsoft\Windows NT\CurrentVersion`
 )
 
-var configureWSL = envknob.Bool("TS_DEBUG_CONFIGURE_WSL")
+var configureWSL = envknob.RegisterBool("TS_DEBUG_CONFIGURE_WSL")
 
 type windowsManager struct {
 	logf       logger.Logf
@@ -359,7 +359,7 @@ func (m windowsManager) SetDNS(cfg OSConfig) error {
 
 	// On initial setup of WSL, the restart caused by --shutdown is slow,
 	// so we do it out-of-line.
-	if configureWSL {
+	if configureWSL() {
 		go func() {
 			if err := m.wslManager.SetDNS(cfg); err != nil {
 				m.logf("WSL SetDNS: %v", err) // continue

net/dns/resolver/forwarder.go

@@ -484,13 +484,13 @@ func (f *forwarder) sendDoH(ctx context.Context, urlBase string, c *http.Client,
 	return res, err
 }
 
-var verboseDNSForward = envknob.Bool("TS_DEBUG_DNS_FORWARD_SEND")
+var verboseDNSForward = envknob.RegisterBool("TS_DEBUG_DNS_FORWARD_SEND")
 
 // send sends packet to dst. It is best effort.
 //
 // send expects the reply to have the same txid as txidOut.
 func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDelay) (ret []byte, err error) {
-	if verboseDNSForward {
+	if verboseDNSForward() {
 		f.logf("forwarder.send(%q) ...", rr.name.Addr)
 		defer func() {
 			f.logf("forwarder.send(%q) = %v, %v", rr.name.Addr, len(ret), err)