threader: try to handle pathless messages
Fix encrypted multiparts and attachments indexing

Pull request #483 does not handle encrypted multiparts properly.  It
used to just verify the signature and not process the parts of the
multipart.  This commit resolves this issue.

Additionally it did not index attachments properly and in the case of a
multipart directly containing more than one multiparts resulted on non
unique indexing of attachments/parts.  This commit resolves this issue
as well.
Remove left over argument from merge