MINOR: mux-quic: factorize send subscribing

MINOR: mux-quic: factorize send subscribing

Factorize code for send subscribing on the lower layer in a dedicated
function qcc_subscribe_send(). This allows to call the lower layer only
if not already subscribed and print a trace in this case. This should
help to understand when subscribing is really performed.

In the future, this function may be extended to avoid subscribing under
new conditions, such as connection already on error.

This should be backported up to 2.7.