cmd/syncthing: Use a 10 second EWMA for CPU usage, refactor a little

We have it lying around so might as well use it instead of inventing our
own thing.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4111

cmd/syncthing/cpuusage_unix.go

@@ -0,0 +1,18 @@
+// Copyright (C) 2014 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/.
+
+//+build !windows,!solaris
+
+package main
+
+import "syscall"
+import "time"
+
+func cpuUsage() time.Duration {
+	var rusage syscall.Rusage
+	syscall.Getrusage(syscall.RUSAGE_SELF, &rusage)
+	return time.Duration(rusage.Utime.Nano() + rusage.Stime.Nano())
+}