T6169: DNS forwarding should allow underscore for srv record

This srv recors looks valid:
```
set service dns forwarding authoritative-domain _tcp.db.mongors1.example.com records srv _mongodb entry 0 hostname 'mongors1.example.com'
```
But FQDN validator cannot validate it correctly, use regex to fix

interface-definitions/service_dns_forwarding.xml.in

@@ -115,7 +115,7 @@
                     <description>An absolute DNS domain name</description>
                   </valueHelp>
                   <constraint>
-                    <validator name="fqdn"/>
+                    <regex>((?!-)[-_a-zA-Z0-9.]{1,63}|@|any)(?&lt;!\.)</regex>
                   </constraint>
                 </properties>
                 <children>