controlclient: log server key and URL

Turns out we never reliably log the control plane URL a client connects
to. Do it here, and include the server public key, which might
inadvertently tell us something interesting some day.

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

control/controlclient/direct.go

@@ -354,6 +354,7 @@ func (c *Direct) doLogin(ctx context.Context, opt loginOpt) (mustRegen bool, new
 		if err != nil {
 			return regen, opt.URL, err
 		}
+		c.logf("control server key %s from %s", serverKey.HexString(), c.serverURL)
 
 		c.mu.Lock()
 		c.serverKey = serverKey