device: allow reducing queue constants on iOS

Heavier network extensions might require the wireguard-go component to
use less ram, so let users of this reduce these as needed.

At some point we'll put this behind a configuration method of sorts, but
for now, just expose the consts as vars.

Requested-by: Josh Bleecher Snyder <josh@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/device.go

@@ -210,7 +210,7 @@ func (device *Device) Down() error {
 func (device *Device) IsUnderLoad() bool {
 	// check if currently under load
 	now := time.Now()
-	underLoad := len(device.queue.handshake.c) >= UnderLoadQueueSize
+	underLoad := len(device.queue.handshake.c) >= QueueHandshakeSize/8
 	if underLoad {
 		atomic.StoreInt64(&device.rate.underLoadUntil, now.Add(UnderLoadAfterTime).UnixNano())
 		return true