Continue a non-blocking HTTP request/response exchange on rctx.
rctx.
Declared in <openssl/http.h>
<openssl/http.h>
int OSSL_HTTP_REQ_CTX_nbio(OSSL_HTTP_REQ_CTX* rctx);
1 when the exchange completed, -1 when more I/O is needed, or 0 on error.