Fix upload IP check
If no IP addresses are listed at 'upload_ip_nopassword', index.php won't show any login form and indicate the session as authenticated. During the actual upload, script.php will show the error 'Error 2: No password nor allowed IP'. The reason is that jirafeau_challenge_upload_ip returns true if the supplied IP whitelist is empty, but uploading without a password doesn't work this way (and shouldn't). I took the liberty to split 'jirafeau_challenge_upload_ip' into one function for the normal IP check, and one for checking if allowed to upload without a password. Having function names that clearly communicates the intent makes it easier to avoid bugs.
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