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

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.