lib/osutil: Don't chmod in atomic file creation (fixes #2472)

Instead, trust (and test) that the temp file has appropriate permissions
from the start. The only place where this changes our behavior is for
ignores which go from 0644 to 0600. I'm OK with that.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3756

lib/model/model.go

@@ -1248,7 +1248,7 @@ func (m *Model) SetIgnores(folder string, content []string) error {
 
 	path := filepath.Join(cfg.Path(), ".stignore")
 
-	fd, err := osutil.CreateAtomic(path, 0644)
+	fd, err := osutil.CreateAtomic(path)
 	if err != nil {
 		l.Warnln("Saving .stignore:", err)
 		return err