MINOR: mux-h2/trace: report a few connection-level info during h2_init()

MINOR: mux-h2/trace: report a few connection-level info during h2_init()

It is currently very difficult to match some H2 trace outputs against
some log extracts because there's no exactly equivalent info.

This patch tries to address this by adding a TRACE_USER() call in h2_init()
that is matched in h2_trace() to report:
  - connection pointer and direction
  - frontend's name or server's name
  - transport layer and control layer (e.g. "SSL/tcpv4")
  - source and/or destination depending on what is set

This now permits to get something like this at verbosity level complete:

  <0>2021-06-16T18:30:19.810897+02:00 [00|h2|1|mux_h2.c:1006] new H2 connection : h2c=0x19fee50(F,PRF) : conn=0x7f373c026850(IN) fe=h2gw RAW/tcpv4 src=127.0.0.1:19540
  <0>2021-06-16T18:30:19.810919+02:00 [00|h2|1|mux_h2.c:2731] rcvd H2 request  : h2c=0x19fee50(F,FRH)
  <0>2021-06-16T18:30:19.810998+02:00 [00|h2|1|mux_h2.c:1006] new H2 connection : h2c=0x1a04ee0(B,PRF) : conn=0x1a04ce0(OUT) sv=h2gw/s1 RAW/tcpv4 dst=127.0.0.1:4446