MINOR: qmux: change API for snd_buf FIN transmission

MINOR: qmux: change API for snd_buf FIN transmission

Previous patches have fixes interim response encoding via
h3_resp_headers_send(). However, it is still necessary to adjust h3
layer state-machine so that several successive HTTP responses are
accepted for a single stream.

Prior to this, QMUX was responsible to decree that the final HTX message
was encoded so that FIN stream can be emitted. However, with interim
response, MUX is in fact unable to properly determine this. As such,
this is the responsibility of the application protocol layer. To reflect
this, app_ops snd_buf callback is modified so that a new output argument
<fin> is added to it.

Note that for now this commit does not bring any functional change.
However, it will be necessary for the following patch. As such, it
should be backported prior to it to every versions as necessary.