ipn/ipnlocal: allow connecting to local web client

The local web client has the same characteristic as tailscale serve, in
that it needs a local listener to allow for connections from the local
machine itself when running in kernel networking mode.

This change renames and adapts the existing serveListener to allow it to
be used by the web client as well.

Updates tailscale/corp#14335

Signed-off-by: Will Norris <will@tailscale.com>

ipn/ipnlocal/web_client_stub.go

@@ -27,3 +27,4 @@ func (b *LocalBackend) WebClientShutdown() {}
 func (b *LocalBackend) handleWebClientConn(c net.Conn) error {
 	return errors.New("not implemented")
 }
+func (b *LocalBackend) updateWebClientListenersLocked() {}