Filter makeCredential() by acceptable trust anchors

[jyasskin]

https://w3c.github.io/webauthn/#registering-a-new-credential says that a relying party has "a list of acceptable trust anchors", and it only accepts attestations rooted at those anchors. However, I don't see any way to tell the UA to only show the user a choice of authenticators that can produce such acceptable attestations. If I haven't just missed that option, it seems like this is going to produce frustrated users who select an authenticator that it turns out the site won't accept.

[equalsJeffH]

on 29-Nov-2017 webauthn call: we have consensus that we do not wish to add RP-driven attstn stmt format ident selection as a part of authnr selection, and so are closing this issue and PR #683.