Finally fix predicted steps smoothing.

Properly set predicted_step_frame to avoid double steps. Only bottom
clamp to server returned data when there is an actual prediction error.

Normal steps can be as low as 127 units due to rounding errors. Account
for that when deciding how fast the step needs to be.
Display loading status at console bottom line.
Mark Con_Printf with q_printf attribute.
Don't allow inflateInit2() access download data.

It was a bad idea to allow since corrupted input could result in fatal
error.