The general procedure to use this module is:
Present the fields to the user, and obtain their submission.
Post to this module, supplying $1returnurl and any relevant fields.
Check the status in the response.
* If you received PASS or FAIL, you're done. The operation either succeeded or it didn't.
* If you received UI, present the new fields to the user and obtain their submission. Then post to this module with $1continue and the relevant fields set, and repeat step 4.
* If you received REDIRECT, direct the user to the redirecttarget and wait for the return to $1returnurl. Then post to this module with $1continue and any fields passed to the return URL, and repeat step 4.
* If you received RESTART, that means the authentication worked but we don't have a linked user account. You might treat this as UI or as FAIL.