cmd/syncthing: Implement log rotation (fixes #6104) (#6198)

Since we've taken upon ourselves to create a log file by default on
Windows, this adds proper management of that log file. There are two new
options:

  -log-max-old-files="3"    Number of old files to keep (zero to keep only current).
  -log-max-size="10485760"  Maximum size of any file (zero to disable log rotation).

The default values result in four files (syncthing.log, synchting.0.log,
..., syncthing.3.log) each up to 10 MiB in size. To not use log rotation
at all, the user can say --log-max-size=0.

cmd/syncthing/main.go

@@ -154,6 +154,8 @@ type RuntimeOptions struct {
 	browserOnly      bool
 	hideConsole      bool
 	logFile          string
+	logMaxSize       int
+	logMaxFiles      int
 	auditEnabled     bool
 	auditFile        string
 	paused           bool
@@ -180,6 +182,8 @@ func defaultRuntimeOptions() RuntimeOptions {
 		cpuProfile:   os.Getenv("STCPUPROFILE") != "",
 		stRestarting: os.Getenv("STRESTART") != "",
 		logFlags:     log.Ltime,
+		logMaxSize:   10 << 20, // 10 MiB
+		logMaxFiles:  3,        // plus the current one
 	}
 
 	if os.Getenv("STTRACE") != "" {
@@ -222,6 +226,8 @@ func parseCommandLineOptions() RuntimeOptions {
 	flag.BoolVar(&options.paused, "paused", false, "Start with all devices and folders paused")
 	flag.BoolVar(&options.unpaused, "unpaused", false, "Start with all devices and folders unpaused")
 	flag.StringVar(&options.logFile, "logfile", options.logFile, "Log file name (still always logs to stdout). Cannot be used together with -no-restart/STNORESTART environment variable.")
+	flag.IntVar(&options.logMaxSize, "log-max-size", options.logMaxSize, "Maximum size of any file (zero to disable log rotation).")
+	flag.IntVar(&options.logMaxFiles, "log-max-old-files", options.logMaxFiles, "Number of old files to keep (zero to keep only current).")
 	flag.StringVar(&options.auditFile, "auditfile", options.auditFile, "Specify audit file (use \"-\" for stdout, \"--\" for stderr)")
 	flag.BoolVar(&options.allowNewerConfig, "allow-newer-config", false, "Allow loading newer than current config version")
 	if runtime.GOOS == "windows" {