Created attachment 222452 [details] [review]
Fix that handles data connections in IPv6 FTP sessions correctly

When connecting to a IPv6 FTP server, any data connection in passive mode is not possible. Even the initial GET_CWD is not working. Below you find the client-server chat of the FTP session. After the LIST -L instruction nothing happens.
On the server you can find a new listening socket on the port in the Extended Passive Mode message.
                                       
220 ProFTPD Server (Debian) [2a01:xxxx:121:xxxx:0:58c6:901e:1]
USER juergen

331 Password required for alex
PASS xxxx
230 User alex logged in
SYST

215 UNIX Type: L8
TYPE I

200 Type set to I
PWD

257 "/" is the current directory
Lade Verzeichnisliste / von der Gegenstelle (LC_TIME=de_DE.UTF-8)
EPSV

229 Entering Extended Passive Mode (|||48620|)
LIST -L

This behaviour is due to incorrect handling of the different Address Families when creating new sockets for the data connections.

The attached patch fixes the issue and dataconnections are as well possible for IPv6 FTP sessions.

The patch has to be appied appying the command `patch -p2 < /path/to/gftp-ipv6-dataconn.patch` in diretory "gftp-2.0.19/lib".