save_uploaded_file() read, and prune_images() deleted, any path in the
"uploaded" POST field. Uploads now go to UPLOAD_DIR under a random name
that only the session knows.
Replace the short open tags and the removed functions (mcrypt, mysql_error,
salt-less crypt). Decode the reCAPTCHA reply as JSON, turn off mysqli
exceptions, drop stripslashes(), and default missing input values.
Fix the misplaced continue in prune_images().
tim