Disable FTP Bounce (FXP) support by default

The Synchronet FTP server has (since 2001) disallowed PORT/EPRT/LPRT commands with a "reserved" port number (i.e. < 1024) as recommended by RFC2577 and when attempted, would log a "SUSPECTED FTP BOUNCE HACK ATTEMPT" in the data/hack.log file.

However, as Karloch (HISPAMSX) pointed out recently, an FTP Bounce Attack to other TCP ports was still possible (and detected/reported by some security scans as a potential vulnerability).

So, reject all PORT/EPRT/LPRT commands that specify an IP address other than that used for the control TCP connection unless the sysop specifically enables the new "ALLOW_BOUNCE" option flag (in the [ftp] section of sbbs.ini) and the user is an authenticated non-guest/anonymous user. And as before, log the attempt as a suspected hack attempt.

This change also removes the "Directory File Access" checkbox from the Synchronet Control Panel for Windows as that feature is "going away" soon (or at least, it won't be an FTP-specific option/feature if it remains).