MINOR: mux-h2: extract the code to send preface+settings into its own function

MINOR: mux-h2: extract the code to send preface+settings into its own function

The code that deals with sending preface + settings and changing the
state currently is in h2_process_mux(), but we'll want to do it as
well from h2_snd_buf(), so let's move it to a dedicate function first.
At this point there is no functional change.