Add SSL_CTX_get_ciphers()

Add an accessor for SSL_CTX.

Since libssl was made opaque, there is no way for users to access the
cipher_list, while users can set the cipher_list by
SSL_CTX_set_cipher_list().

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>