MEDIUM: xprt_qstrm: implement QMux record parsing

MEDIUM: xprt_qstrm: implement QMux record parsing

This patch implements the new QMux record layer parsing for xprt_qstrm.
This is mostly similar to the MUX code from the previous patch.

Along with this change, a new xprt_qstrm layer accessor exposes the
possible remaining record length after Transport parameters parsing.
This can only occur when xprt_qstrm Rx buffer is not completely emptied
due to other following frames. If stored in the same record, MUX layer
has to know the remaining record length.

Thus, xprt_qstrm_rxrlen() is now used in qmux_init() to preinitialize
<rx.rlen> QCC field.