BUG/MINOR: session: Emit an HTTP error if accept fails only for H1 connection

If session_accept_fd() fails for a raw HTTP socket, we try to send an HTTP error
500. But, we must also take care it is an HTTP/1 connection. We cannot rely on
the mux at this stage, because the error, if any, happens before or during its
creation. So, instead, we check if the mux_proto is specified or not. Indeed,
the mux h1 cannot be forced on the bind line and there is no ALPN to choose
another mux on a raw socket. So if there is no mux_proto defined for a raw HTTP
socket, we are sure to have an HTTP/1 connection.

This patch must be backported to 2.0 and 1.9.

(cherry picked from commit 76f4c370f149149c07bf1f88ae10c33c36c458a0)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/session.c b/src/session.c
index 0ce1c4e..f0f2b71 100644
--- a/src/session.c
+++ b/src/session.c
@@ -300,7 +300,8 @@
 	conn_free(cli_conn);
  out_close:
 	listener_release(l);
-	if (ret < 0 && l->bind_conf->xprt == xprt_get(XPRT_RAW) && p->mode == PR_MODE_HTTP) {
+	if (ret < 0 && l->bind_conf->xprt == xprt_get(XPRT_RAW) &&
+	    p->mode == PR_MODE_HTTP && l->bind_conf->mux_proto == NULL) {
 		/* critical error, no more memory, try to emit a 500 response */
 		struct buffer *err_msg = &p->errmsg[HTTP_ERR_500];
 		if (!err_msg->area)