[libcvd-members] GUI/GVars readline code

[Georg Klein]

Would anyone be terribly upset if I had a go at changing the GUI libreadline 
call? IMO the current model is bizarre, and the whole thing could be 
replaced by a singleton which handles interactive console input, using 
libreadline if it's available and std::getline if it's not.

This is partially motivated by the fact that libreadline is GPL, so having 
it optional is a good thing if gvars is supposed to be LGPL!