MEDIUM: mux_quic: rename qmux traces to qcm

MEDIUM: mux_quic: rename qmux traces to qcm

This patch is part of a renaming affecting mux_quic and related files.
Naming "qcm" will become the new marker for common mux_quic stuffs,
shared both on QUIC and QMux protocols. The final objective is to limit
"qmux" naming for stuffs related to the new experimental protocol of the
same name.

The current patch renames mux_quic traces token to "qcm". This is the
only change with external visible impacts in the whole renaming series.
However, to preserve compatibility, trace source alias is defined with
the older name "qmux". This relies on the previous patch which
introduced "alias" new trace_source member.