Design doc attachments cannot start with an underscore

We throw an error if someone tries to request an attachment like that:

https://github.com/apache/couchdb/commit/347015

I think the following fix might be preferable:

https://github.com/cloudant/chttpd/commit/741a83

It'll send a 404 if the user tries an unknown action, unless there happens to be an attachment with that same name. I suppose we could also just disallow attachment names that start with an underscore, like we do for document IDs.