MINOR: stream-int: Notify mux when the buffer is not stuck when calling rcv_buf

MINOR: stream-int: Notify mux when the buffer is not stuck when calling rcv_buf

The transient flag CO_RFL_BUF_NOT_STUCK should now be set when the mux's
rcv_buf() function is called, in si_cs_recv(), to be sure the mux is able to
perform some optimisation during data copy. This flag is set when we are
sure the channel buffer is not stuck. Concretely, it happens when there are
data scheduled to be sent.

It is not a fix and this flag is not used for now. But it makes sense to have
this info to be sure to be able to do some optimisations if necessary.

This patch is related to the issue #1362. It may be backported to 2.4 to
ease future backports.