Detect when KMess can become the direct conneciton server

[diederik]

We need to add some check if the system can accept incoming connections.

Possible options:

• automatic check using STUN, TURN or UPnP.
• add a checkbox to a global "file transfer settings" panel so a system admin can disable server sockets because he knows his network setup won't be able to handle it anyway.

This setting can also be used to disable "direct connections" for MsnObject transfers. If you're behind a closed firewall, you'll have to wait for a 1 minute timeout before the data is sent. It would be better if the direct connection attempt is rejected straight away, because it's already known the DC setup won't work.