resolved: keep parsing DNS delegate servers after duplicates

The DNS delegate parser tries to filter duplicate DNS= entries, but the
duplicate branch returned from the whole parser. This made a duplicate
server truncate the rest of the same DNS= line, silently dropping later
valid servers.

Reproducer:

  DNS=192.168.77.78 192.168.77.78 192.168.77.79

Before: only 192.168.77.78 appeared in the delegate server list.

Follow-up for: 7928c0e0a1

src/resolve/resolved-dns-delegate.c

@@ -281,7 +281,7 @@ int config_parse_delegate_dns_servers(
                         /* Drop the marker. This is used to find the servers that ceased to exist, see
                          * manager_mark_dns_servers() and manager_flush_marked_dns_servers(). */
                         dns_server_move_back_and_unmark(s);
-                        return 0;
+                        continue;
                 }
 
                 r = dns_server_new(