Refactor tools code
    
    Move parameter parsing in tools before attempting to do anything that
    might fail - so that we have debug_level set correctly for potential
    error messages. That allows printing the --help and --usage messages
    without being root.
    
    Fix code duplicates in tools and refactor its code a little to lay
    ground for decoupling the synchronous interfaces.
    
    Remove some legacy tools leftovers, re-add sensible error message on
    removing nonexistent users/groups which was removed by accident.
    
    Fixes: Trac ticket #75
    Fix typo in groupdel: fixes ticket #136