Be more careful about cleaning up attachments

lisp/gnorb-gnus.el: Function `gnorb-gnus-capture-attach' should set
		    `gnorb-gnus-capture-attachments' to nil when it's
		    done, now that more functions are accessing
		    attachments.
Return visit candidates correctly.

lisp/gnorb-org.el: Function`gnorb-org-find-visit-candidates' should
		   append, not push -- all return values are now lists.