MINOR: quic: refactor application send

MINOR: quic: refactor application send

Adjust qc_send_app_pkts function : remove <old_data> arg and provide a
new wrapper function qc_send_app_probing() which should be used instead
when probing with old data.

This simplifies the interface of the default function, most notably for
the MUX which does not interfer with retransmission.
QUIC_FL_CONN_RETRANS_OLD_DATA flag is set/unset directly in the wrapper
qc_send_app_probing().

At the same time, function documentation has been updated to clarified
arguments and return values.

This commit will be useful for the next patch to differentiate MUX and
retransmission send context. As a consequence, the current patch should
be backported wherever the next one will be.