#8 URL scanning on connect

[Pavel Orehov]

Scan URL in URLCategorization engines when FTP
client connects to FTP proxy.
It may execute the same executable for virus scanning
with same parameters, but executable will ignore the
scan filename that is not actually in this case.

Details:
- FTP client connects to FTP proxy
- FTP client send USER command with requested FTP
server name (e.g. ftp.playboy.com).
- Frox call to scan executable with same parameters as
virus scanner, but with empty string scan filename
- If executable returns VSOK Frox continue the session
with FTP client, otherwise it send client some error
message and close the connection