critpath.py: Refactor main execution path
    
    This avoids doing all the main execution stuff at global scope,
    and splits out argument parsing to make the new main() function
    a bit smaller. We also use argparse's built-in ability to set
    mutually-exclusive arguments instead of reinventing it.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>