wgengine/magicsock: move debug knobs to their own file, compile out on iOS

No need for these knobs on iOS where you can set the environment
variables anyway.

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

wgengine/magicsock/debugknobs_ios.go

@@ -0,0 +1,20 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package magicsock
+
+// All knobs are disabled on iOS.
+// Further, they're const, so the toolchain can produce smaller binaries.
+const (
+	logPacketDests          = false
+	debugDisco              = false
+	debugOmitLocalAddresses = false
+	debugUseDerpRouteEnv    = ""
+	debugUseDerpRoute       = false
+	logDerpVerbose          = false
+	debugReSTUNStopOnIdle   = false
+	debugAlwaysDERP         = false
+)
+
+func inTest() bool { return false }