MINOR: mux-h1: Always subscribe for reads when splicing is disabled

MINOR: mux-h1: Always subscribe for reads when splicing is disabled

In h1_rcv_pipe(), when the splicing is not possible or disabled at the end
of the fnuction, we make sure to subscribe for reads. It is not a bug but it
avoid an extra call to h1_rcv_pipe() to handle the subscription in some
cases (end of message, end of chunk or read0).

In addition, the condition to detect end of splicing has been simplified. We
now only rely on H1C_F_WANT_SPLICE flags.