this script use an $op to know what to do.
if $op is empty or none of the above values,
- the default screen is build (with all records, or filtered datas).
- the user can clic on add, modify or delete record.
- if primkey exists, this is a modification,so we read the $primkey record
- builds the add/modify form
- the user has just send datas, so we create/modify the record
- we show the record having primkey=$primkey and ask for deletion validation form
- we delete the record having primkey=$primkey