Optionally check for early data

This adds a way to use the last byte of the buffer to change the
behavior of the server. The last byte is used so that the existing
corpus can be reused either without changing it, or just adding a single
byte, and that it can still be used by other projects.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
GH: #2683