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_darwin.go

@@ -0,0 +1,28 @@
+package restic
+
+import (
+	"os/user"
+	"strconv"
+)
+
+func (sn *Snapshot) fillUserInfo() error {
+	usr, err := user.Current()
+	if err != nil {
+		return err
+	}
+
+	sn.Username = usr.Username
+	uid, err := strconv.ParseInt(usr.Uid, 10, 32)
+	if err != nil {
+		return err
+	}
+	sn.UID = uint32(uid)
+
+	gid, err := strconv.ParseInt(usr.Gid, 10, 32)
+	if err != nil {
+		return err
+	}
+	sn.GID = uint32(gid)
+
+	return nil
+}