Add SSL_CTX_set1_cert_store()

For convenience, combine getting a new ref for the new SSL_CTX
with assigning the store and freeing the old one.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1755)