MINOR: mux-h2/traces: clarify the "rejected H2 request" event

MINOR: mux-h2/traces: clarify the "rejected H2 request" event

In h2_frt_handle_headers() all failures lead to a generic message saying
"rejected H2 request". It's quite inexpressive while there are a few
distinct tests that are made before jumping there:

  - trailers on closed stream
  - unparsable request
  - refused stream

Let's emit the traces from these call points instead so that we get more
info about what happened. Since these are user-level messages, we take
care of keeping them aligned as much as possible.

For example before it would say:

  [04|h2|1|mux_h2.c:2859] rejected H2 request : h2c=0x7f5d58036fd0(F,FRE)
  [04|h2|5|mux_h2.c:2860] h2c_frt_handle_headers(): leaving on error : h2c=0x7f5d58036fd0(F,FRE) dsi=1 h2s=0x9fdb60(0,CLO)

And now it says:

  [04|h2|1|mux_h2.c:2817] rcvd unparsable H2 request : h2c=0x7f55f8037160(F,FRH) dsi=1 h2s=CLO
  [04|h2|5|mux_h2.c:2875] h2c_frt_handle_headers(): leaving on error : h2c=0x7f55f8037160(F,FRE) dsi=1 h2s=CLO