MINOR: mux-h1: Add functions to send HTTP errors from the mux

MINOR: mux-h1: Add functions to send HTTP errors from the mux

For now, this part is unsued. But this patch adds functions to handle errors
on idle and embryonic H1 connections and send corresponding HTTP error
messages to the client (400, 408 or 500). Thanks to previous patches, these
functions take care to update the right stats counters, but also the
counters tracked by the session.

A field to store the HTTP error code has been added in the H1C structure. It
is used for error retransmits, if any, and to get it in http logs. It is
used to return the mux exit status code when the MUX_EXIT_STATUS ctl
parameter is requested.