rfe: allow @ (at symbol) as DNS hostname

Feature request: Right now ISPConfig ui supports DNS records for a domain by leaving the hostname empty, or specifying the full domain.tld. with trailing dot. Those familiar with BIND config tend to use an at symbol @ to mean this as well, which results in an error. It would be simple to convert a single @ hostname to an empty string (or to the domain name, like the dns importer does) either in javascript when validating, or in php when saving.