List Checkbox Default "Checked" Cannot Be Unchecked On Node Form

Issue Summary

SimpleTest claims another new bug: When creating a field, if the "List" field is enabled and defaults to "List files by default", then the checkbox is impossible to uncheck on the node form. Editing a node and unchecking the "List" box is not saved, the file is still visible, and re-editing the node the checkbox will be checked again.

This is because an empty checkbox does not pass any value to CCK, so we must check for the lack of a value at all, rather than expecting "0". This patch checks if a "List" field is in a file field, then sets the value to "0" if the list value is missing entirely.