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/main.go

@@ -19,6 +19,7 @@ import (
 	_ "net/http/pprof"
 	"net/url"
 	"os"
+	"os/signal"
 	"path/filepath"
 	"regexp"
 	"runtime"
@@ -26,6 +27,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
 
 	"github.com/syncthing/syncthing/lib/config"
@@ -462,6 +464,8 @@ func upgradeViaRest() error {
 }
 
 func syncthingMain() {
+	setupSignalHandling()
+
 	// Create a main service manager. We'll add things to this as we go along.
 	// We want any logging it does to go through our log system.
 	mainSvc := suture.New("main", suture.Spec{
@@ -839,6 +843,28 @@ func syncthingMain() {
 	os.Exit(code)
 }
 
+func setupSignalHandling() {
+	// Exit cleanly with "restarting" code on SIGHUP.
+
+	restartSign := make(chan os.Signal, 1)
+	sigHup := syscall.Signal(1)
+	signal.Notify(restartSign, sigHup)
+	go func() {
+		<-restartSign
+		stop <- exitRestarting
+	}()
+
+	// Exit with "success" code (no restart) on INT/TERM
+
+	stopSign := make(chan os.Signal, 1)
+	sigTerm := syscall.Signal(15)
+	signal.Notify(stopSign, os.Interrupt, sigTerm)
+	go func() {
+		<-stopSign
+		stop <- exitSuccess
+	}()
+}
+
 // printHashRate prints the hashing performance in MB/s, formatting it with
 // appropriate precision for the value, i.e. 182 MB/s, 18 MB/s, 1.8 MB/s, 0.18
 // MB/s.