allow subnets in bind notify/allow-transfer

From https://www.howtoforge.com/community/threads/bug-in-notify-and-allow-transfer.75122/

According to Bind9 it is allowed to use subnets.

So instead of using: 192.168.0.1,192.168.0.2,192.168.0.3,192.168.0.4,192.168.0.5,.... You could use 192.168.0.0/28

But ISPConfig doesn't allow this.