Introduce UserID, GroupID for snapshot

Use the fields UID and GID for Unix user and group ID, and the fields
UserID and GroupID for Windows user and group strings.

snapshot_windows.go

@@ -0,0 +1,16 @@
+package restic
+
+import "os/user"
+
+func (sn *Snapshot) fillUserInfo() error {
+	usr, err := user.Current()
+	if err != nil {
+		return err
+	}
+
+	sn.Username = usr.Username
+	sn.UserID = usr.Uid
+	sn.GroupID = usr.Gid
+
+	return nil
+}