cChannel::Nid() should be renamed to cChannel::Onid()

[patrick_fischer at gmx.de (Patrick Fischer)]

The Function cChannel::Nid() returns the OridignalNetworkID and not the 
NetworkID, so it is a little bit confusing.
This are two differents Values.
So the cChannel::Nid() should return the NetworkID and cChannel::Onid() 
should return the OriginalNetworkID.
I know vdr will not use this values, but I do.

Greetings
Patrick