qc_rcv_buf and qc_snd_buf are names used for static functions in both
quic-sock and quic-mux. To remove this ambiguity, slightly modify names
used in MUX code.
In the future, we should properly define a unique prefix for all QUIC
MUX functions to avoid such problem in the future.
This change is part of quic-conn owned socket implementation.
It may be backported to 2.7 after a period of observation.