dns: check NS entry to be used as primary NS

Don't just blindly use the first custom NS entry for SOA, actually check
if it pertains to the domain in question

lib/Froxlor/Dns/Dns.php

@@ -181,8 +181,8 @@ class Dns
 				// unset special spf required-entry
 				unset($required_entries[$entry['type']][md5("@SPF@")]);
 			}
-			if (empty($primary_ns) && $entry['type'] == 'NS') {
-				// use the first NS entry as primary ns
+			if (empty($primary_ns) && $entry['record'] == '@' && $entry['type'] == 'NS') {
+				// use the first NS entry pertaining to the current domain as primary ns
 				$primary_ns = $entry['content'];
 			}
 			// check for CNAME on @, www- or wildcard-Alias and remove A/AAAA record accordingly