MINOR: h3/qmux: Set QC_SF_UNKNOWN_PL_LENGTH flag on QCS when headers are sent

MINOR: h3/qmux: Set QC_SF_UNKNOWN_PL_LENGTH flag on QCS when headers are sent

QC_SF_UNKNOWN_PL_LENGTH flag is set on the qcs to know a payload of message
has an unknown length and not send a RESET_STREAM on shutdown. This flag was
based on the HTX extra field value. However, it is not necessary. When
headers are processed, before sending them, it is possible to check the HTX
start-line to know if the length of the payload is known or not.

So let's do so and don't use anymore the HTX extra field for this purpose.