Tweaks for client certs

Two changes here, one editorial and one mostly-editorial:
  - The endpoint might send a connection *or* a stream error of HTTP_1_1_REQUIRED, depending on the scope of the requirement.  Note that this was non-normative language, so removing the implication that it would always be a connection error simply makes clear what was already permitted.
  - The server might know as soon as the connection is established that it will want the cert, but want it protected by the handshake.  Either endpoint, not just clients, may choose to renegotiate before sending their preface.