Add osutil.AtomicWriter

This captures the common pattern of writing to a temp file and moving it
to it's real name only if everything went well. It reduces the amount of
code in some places where we do this, but maybe not as much as I
expected because the upgrade thing is still a special snowflake...

internal/model/model.go

@@ -13,7 +13,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net"
 	"os"
 	"path/filepath"
@@ -936,30 +935,17 @@ func (m *Model) SetIgnores(folder string, content []string) error {
 		return fmt.Errorf("Folder %s does not exist", folder)
 	}
 
-	fd, err := ioutil.TempFile(cfg.Path(), ".syncthing.stignore-"+folder)
+	fd, err := osutil.CreateAtomic(filepath.Join(cfg.Path(), ".stignore"), 0644)
 	if err != nil {
 		l.Warnln("Saving .stignore:", err)
 		return err
 	}
-	defer os.Remove(fd.Name())
 
 	for _, line := range content {
-		_, err = fmt.Fprintln(fd, line)
-		if err != nil {
-			l.Warnln("Saving .stignore:", err)
-			return err
-		}
+		fmt.Fprintln(fd, line)
 	}
 
-	err = fd.Close()
-	if err != nil {
-		l.Warnln("Saving .stignore:", err)
-		return err
-	}
-
-	file := filepath.Join(cfg.Path(), ".stignore")
-	err = osutil.Rename(fd.Name(), file)
-	if err != nil {
+	if err := fd.Close(); err != nil {
 		l.Warnln("Saving .stignore:", err)
 		return err
 	}