MINOR: xprt_qmux: use qmux instead of qstrm naming

MINOR: xprt_qmux: use qmux instead of qstrm naming

This is a follow-up on the QUIC MUX renaming process.

The current patch performs renaming in xprt_qmux layer. Older "qstrm"
identifier is replaced by the new name "qmux". Every remaining functions
and structures in xprt_qmux are changed. Outside effects are only
present in QUIC MUX which directly uses some of these functions.