mode=checkcaptcha only works on some boards!
Until a proper api exists for checking what boards have this option, it will not be implemented.
(that is, a proper way to check if "var preupload_captcha = true" was set)


Send to the normal url "https://sys.4chan.org/" + board + "/post";
Used in the extension for checking if the captcha is correct when submitting a large file

POST:
mode=checkcaptcha
challenge=<normal captcha challenge>
response=<user response>


returns json:
{
    "token": "<captcha token>",
    "error": "<error shown to the user>",
    "fail": "<fail logged to console>"
}

In the extension:

if (response.token) {
    a = $.id("qrCapToken"), 
    a.value = response.token, 
    a.removeAttribute("disabled"), 
    QR.submitDirect()
} else {
    if (response.error) {
        QR.reloadCaptcha(), 
        QR.btn.value = "Post", 
        QR.showPostError(response.error)) 
    } else {
        if (response.fail) {
            console.log(b.fail), 
            QR.submitDirect()
        }
    }
}

qrCapToken is a hidden form field with the name captcha_token
So when token exists in the json, send a normal reply with captcha_token