MINOR: conn-stream: Force mux to wait for read events if abortonclose is set

MINOR: conn-stream: Force mux to wait for read events if abortonclose is set

When the abortonclose option is enabled, to be sure to be immediately
notified when a shutdown is received from the client, the frontend
conn-stream must be sure the mux will wait for read events. To do so, the
CO_RFL_KEEP_RECV flag is set when mux->rcv_buf() is called. This new flag
instructs the mux to wait for read events, regardless its internal state.

This patch is required to fix abortonclose option for H1 client connections.

(cherry picked from commit d8219b31e7cfe0dec64c33e1186e5f33cf87191a)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>