Currently, we set interactive mode as default possibly reverting the
setting after we parse the command line arguments. There's nothing
particulary wrong with that, but a call to vshReadlineInit is performed
always in the global initializer just because the default mode is interactive.
Rather than moving vshReadlineInit call somewhere else (because another client
might want to implement interactive mode only), we could make the decision
if we're about to run in interactive mode once the command line is parsed.