MINOR: mux-quic: provide a "show_sd" helper to output stream debugging info

MINOR: mux-quic: provide a "show_sd" helper to output stream debugging info

It's very limited but at least provides the very basic info about QCS and
QCC when issuing "show sess all":

  scf=0x7fa9642394a0 flags=0x00000080 state=EST endp=CONN,0x7fa9642351f0,0x02001001 sub=3
>     qcs=0x7fa9642351f0 .flg=0x5 .id=396 .st=HCR .ctx=0x7fa9642353f0, .err=0
>     qcc=0x7fa96405ce20 .flg=0 .nbsc=100 .nbhreq=100, .task=0x7fa964054260
      co0=0x7fa96405cd50 ctrl=quic4 xprt=QUIC mux=QUIC data=STRM target=LISTENER:0x328c530
      flags=0x00200300 fd=-1 fd.state=00 updt=0 fd.tmask=0x0

It will need to be improved but it's better than nothing already. This
should be backported to 2.6 if the other dumps are backported.