BUG/MINOR: mux-h1: Don't rely on CO_FL_SOCK_RD_SH to set H1C_F_CS_SHUTDOWN

BUG/MINOR: mux-h1: Don't rely on CO_FL_SOCK_RD_SH to set H1C_F_CS_SHUTDOWN

The CO_FL_SOCK_RD_SH flag is only set when a read0 is received. So we must not
rely on it to set the H1 connection in shutdown state (H1C_F_CS_SHUTDOWN). In
fact, it is suffisant to set the connection in shutdown state when the shutdown
for writes is forwared to the sock layer.

This patch must be backported as far as 1.9.