(SERVER-18) Add x-client-dn and x-client-verify support

It may make sense to go ahead and set `:client-cert` to `(:ssl-client-cert request)` if the latter is non-nil at this point.  I think this is analogous to what you'd get with the existing Apache/Passenger behavior.  Not sure about the validity of the use case for specifying the client DN and auth through HTTP headers while still going over SSL with a client certificate, though, so maybe not a big deal.  Suppose this area of the code will change a bit when the work for SERVER-39 is implemented.