/table/add form confusing with labels and foreign keys

If I have a two tables and a simple_crud interface like:

```
create table things ( id int, name varchar, category_id int);
create table categories ( id int, name varchar, );

simple_crud(
prefix=> '/things',
db_table=>'things',
foreign_keys => { strategy_id => { table=>'categories', key_column=>'id', label_column=>'name' }},
labels => { category_id=>"Category" },
);
```

Then when viewing the /things/add form , then for the 'category_id' field of the things table will show an input labelled 'category' even though you have to enter a 'category_id'.

In some cases this is correct (when there's only one row in the categories table, then it seems like it'll show a checkbox for that category), but if asking for a category id this is misleading.

Start by reproducing the /things/add form from the issue using the shown things and categories tables, then trace the simple_crud foreign-key and labels handling. Done means the category_id field's label and input make clear whether the user selects a category or enters an ID, including the single-category case.

Written by the indexing model from the issue text.