MINOR: mux-quic: check fctl during STREAM frame build

MINOR: mux-quic: check fctl during STREAM frame build

qcs_build_stream_frm() is responsible to generate a STREAM frame
pointing to the content of QCS TX buffer.

This patch moves send flow control overflow check from qcs_xfer_data()
to qcs_build_stream_frm(), i.e. from transfer between internal
QCS buffer and qc_stream_desc, to STREAM frame generation.

Flow control is both check at stream and connection level. For
connection flow control, as several frames are built before emission, an
accumulator is used as extra arguments to functions to account the total
length of already built frames.

This patch should not provide any functional changes. Its main purpose
is to prepare for the removal of QCS internal buffer.