Make bulk user picture upload handle subdirectories in zip files (with patch)

The current code for bulk importing user pictures expects the picture files not to be in subdirectories inside the zip file at all. If the zip file has subdirectories inside it, any files inside those subdirectories are ignored as we skip over directories.

The attached patch improves uploadpicture.php to recursively proccess any subdirectories in the zip file (and the files inside them), in addition to processing the files at the 'root' of the zip file.

Saludos. Iñaki.