REORG: mux_quic: use newer qcm prefix for legacy qmux files

REORG: mux_quic: use newer qcm prefix for legacy qmux files

This patch is the first one of the renaming serie, affecting the QUIC
MUX module. The objective is to remove older "qmux" naming which was
used as a generic identifier. Now it should be restricted to the QMux
experimental protocol. A new "qcm" naming will replace the generic
usage.

The current patch renames the files themselves. Token "qmux" is replaced
by the new "qcm" identifier. Makefile and include statements are
adjusted as required.