Improve function portability for processing selections

Right now the `ajax_select()` function handles parsing the `$_REQUEST` global as well as creating the placeholder attachment post, follow up actions and returning a JSON response.

If this functionality were to be required for an import script, or script that converted local attachments to global ones for example, it would have to be recreated wholesale.

We should split `ajax_select()` into 2 functions, one that handles the `$_REQUEST` global and returning JSON, and one that accepts parameters necessary to create the local placeholder attachment posts for global images.