Improve checking for IP address or domain name

[widhalmt]

Thanks @RincewindsHat for pointing that out. There are valid domain names that start with a digit. To be honest, I don't know if a hostname may start with a digit, too according to RFC. But since it's possible, we have a domain name in server.address and client.address we should make sure to have a more robust check in filter-70-client.conf and filter-70-server.conf.

To be honest, I consider host- and domainnames starting with a digit as fairly exotic. So I think it's important to fix that but not an immediate must to start a fix right now.

[RincewindsHat]

Also, it gets worse. I am not quite sure if those fields can contain an IPv6 address, but of course they could start with [a-f]

[widhalmt]

I guess we are not the first to stumble on this. I hope, we'll find some ready to use code.