ssl_servername_cb
SSL_CTX_set_tlsext_servername_callback(ctx, ssl_servername_cb);
static int ssl_servername_cb(SSL *s, int *ad, void *arg);
SSL_CTX_set_tlsext_servername_callback(ctx2, ssl_servername_cb);
SSL_CTX_set_tlsext_servername_callback(ctx, ssl_servername_cb);
static int ssl_servername_cb(SSL *s, int *ad, void *arg);