Spurious HTML generated for nodes with no attachments

When a node has no attachments, the attachment module generates this unnecessary HTML code at the end of the body:

<br /><a name="attachments"></a><div class="attachments"></div>

This causes the excerpt module to generate a "read more" link in the teasers for nodes that should not have that link, and it could cause other similar problems.

The patch fixes this by making sure there is at least one non-hidden attachment before generating any code for the attachment list.