cmd/syncthing: Start CPU usage monitoring not from init (fixes #4183)

Starting stuff from init() is an antipattern, and the innerProcess
variable isn't 100% reliable. We should sort out the other uses of it as
well in due time.

Also removing the hack on innerProcess as I happened to see it and the
affected versions are now <1% users.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4185

cmd/syncthing/mocked_cpuusage_test.go

@@ -0,0 +1,13 @@
+// Copyright (C) 2017 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package main
+
+type mockedCPUService struct{}
+
+func (*mockedCPUService) Rate() float64 {
+	return 42
+}