[PATCH] attachment tree improvement

It turns out that the current code does not handle deeply nested
attachments very well. Attached patch changes the behavior the following
way:

The attachment tree behaves like the thread tree now, so it 'wraps' if
the right edge (minus strlen(txt_attachment_no_name)) is reached. I
adopted the code from thread.c:make_prefix() for this.

Also attached a sample mbox with deeply nested parts for testing.

Dennis