Fold away certificate slots mechanism.

This allows us to remove the confusing EVP_PKEY argument to the
SSL_PRIVATE_KEY_METHOD wrapper functions. It also simplifies some of the
book-keeping around the CERT structure, as well as the API for
configuring certificates themselves. The current one is a little odd as
some functions automatically route to the slot while others affect the
most recently touched slot. Others still (extra_certs) apply to all
slots, making them not terribly useful.

Consumers with complex needs should use cert_cb or the early callback
(select_certificate_cb) to configure whatever they like based on the
ClientHello.

BUG=486295

Change-Id: Ice29ffeb867fa4959898b70dfc50fc00137f01f3
Reviewed-on: https://boringssl-review.googlesource.com/5351
Reviewed-by: Adam Langley <agl@google.com>
11 files changed