restore strip_email_domain for migration

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/util/dns.go

@@ -182,3 +182,33 @@ func GenerateIPv6DNSRootDomain(ipPrefix netip.Prefix) []dnsname.FQDN {
 
 	return fqdns
 }
+
+// TODO(kradalby): Reintroduce when strip_email_domain is removed
+// after #2170 is cleaned up
+// DEPRECATED: DO NOT USE
+// NormalizeToFQDNRules will replace forbidden chars in user
+// it can also return an error if the user doesn't respect RFC 952 and 1123.
+func NormalizeToFQDNRules(name string, stripEmailDomain bool) (string, error) {
+
+	name = strings.ToLower(name)
+	name = strings.ReplaceAll(name, "'", "")
+	atIdx := strings.Index(name, "@")
+	if stripEmailDomain && atIdx > 0 {
+		name = name[:atIdx]
+	} else {
+		name = strings.ReplaceAll(name, "@", ".")
+	}
+	name = invalidCharsInUserRegex.ReplaceAllString(name, "-")
+
+	for _, elt := range strings.Split(name, ".") {
+		if len(elt) > LabelHostnameLength {
+			return "", fmt.Errorf(
+				"label %v is more than 63 chars: %w",
+				elt,
+				ErrInvalidUserName,
+			)
+		}
+	}
+
+	return name, nil
+}