Warn about lack of support for Pure-FTPd in Host Access Control

As documented on the forums, Pure-FTPd does not have TCPWrappers support so Host Access Control (HAC) does not work with the default FTP server. This is confusing and worrying when enabling HAC for ALL or FTP services only to be warned of continued blocked logins by cPHulkd afterwards.

There should be a warning regarding the compatibility of Pure-FTPd on this page with a recommendation to switch to ProFTPd if FTP HAC is desired.