all: Refactor cmd/syncthing creating lib/syncthing (ref #4085) (#5805)

* add skeleton for lib/syncthing

* copy syncthingMain to lib/syncthing (verbatim)

* Remove code to deduplicate copies of syncthingMain

* fix simple build errors

* move stuff from main to syncthing with minimal mod

* merge runtime options

* actually use syncthing.App

* pass io.writer to lib/syncthing for auditing

* get rid of env stuff in lib/syncthing

* add .Error() and comments

* review: Remove fs interactions from lib

* and go 1.13 happened

* utility functions

lib/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 syncthing
+
+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())
+}