Implement signal handling

With this change, the behavior is as follows:

 - SIGTERM / SIGINT: Exit cleanly (exit code 0)
 - SIGHUP: Restart

This is the case both when hitting the monitor process or the inner
process, or if running with NORESTART (but then we won't restart,
obviously, just exit with code exitRestarting).

It enables "pkill -HUP syncthing" to do the right thing to restart all
Syncthings on package upgrade, for example.

cmd/syncthing/monitor.go

@@ -62,9 +62,12 @@ func monitorMain() {
 	args := os.Args
 	var restarts [countRestarts]time.Time
 
-	sign := make(chan os.Signal, 1)
-	sigTerm := syscall.Signal(0xf)
-	signal.Notify(sign, os.Interrupt, sigTerm, os.Kill)
+	stopSign := make(chan os.Signal, 1)
+	sigTerm := syscall.Signal(15)
+	signal.Notify(stopSign, os.Interrupt, sigTerm)
+	restartSign := make(chan os.Signal, 1)
+	sigHup := syscall.Signal(1)
+	signal.Notify(restartSign, sigHup)
 
 	for {
 		if t := time.Since(restarts[0]); t < loopThreshold {
@@ -124,12 +127,17 @@ func monitorMain() {
 		}()
 
 		select {
-		case s := <-sign:
+		case s := <-stopSign:
 			l.Infof("Signal %d received; exiting", s)
 			cmd.Process.Kill()
 			<-exit
 			return
 
+		case s := <-restartSign:
+			l.Infof("Signal %d received; restarting", s)
+			cmd.Process.Signal(sigHup)
+			err = <-exit
+
 		case err = <-exit:
 			if err == nil {
 				// Successful exit indicates an intentional shutdown