MINOR: h3: add traces on decode_qcs callback

MINOR: h3: add traces on decode_qcs callback

Add traces inside h3_decode_qcs(). Every error path has now its
dedicated trace which should simplify debugging. Each early returns has
been converted to a goto invocation.

To complete the demux tracing, demux frame type and length are now
printed using the h3s instance whenever its possible on trace
invocation. A new internal value H3_FT_UNINIT is used as a frame type to
mark demuxing as inactive.

This should be backported up to 2.7.