Harmonise commands for exit in make_ndx and distance
When running make_ndx, the command for save and quit is 'q' and not 'Ctrl-D'. If you press the wrong one it quits with an error.
When running distance, the command for save and quit is 'Ctrl-D' and not 'q'. If you press the wrong one it gives you a warning.
This is counter-intuitive and makes things more difficult. If I'm switching back and forth between the two, I keep using the wrong 'save and quit' command, which is rather frustrating.