CLEANUP: mux-h2/trace: shorten the name of the header enc/dec functions

CLEANUP: mux-h2/trace: shorten the name of the header enc/dec functions

The functions in charge of processing headers have their names in the
traces and they're among the longest of the mux_h2.c file, while even
containing some redundancy. These names are not used outside, let's
shorten them:

- h2c_decode_headers        -> h2c_dec_hdrs
- h2s_bck_make_req_headers  -> h2s_snd_bhdrs
- h2s_frt_make_resp_headers -> h2s_snd_fhdrs

Now the traces are a bit more readable:

  [00|h2|5|mux_h2.c:4822] h2c_dec_hdrs(): h2c=0x1870510(F,FRP) dsi=1 rcvh :method: GET
  [00|h2|5|mux_h2.c:4822] h2c_dec_hdrs(): h2c=0x1870510(F,FRP) dsi=1 rcvh :path: /
  [00|h2|5|mux_h2.c:4822] h2c_dec_hdrs(): h2c=0x1870510(F,FRP) dsi=1 rcvh :scheme: http
  [00|h2|5|mux_h2.c:4822] h2c_dec_hdrs(): h2c=0x1870510(F,FRP) dsi=1 rcvh :authority: localhost:14446
  [00|h2|5|mux_h2.c:4822] h2c_dec_hdrs(): h2c=0x1870510(F,FRP) dsi=1 rcvh user-agent: curl/7.54.1
  [00|h2|5|mux_h2.c:4822] h2c_dec_hdrs(): h2c=0x1870510(F,FRP) dsi=1 rcvh accept: */*