MEDIUM: h2: prevent the various mux encoders from modifying the buffer

MEDIUM: h2: prevent the various mux encoders from modifying the buffer

Functions h2s_frt_make_resp_headers() and h2s_frt_make_resp_data() used
to modify the buffer's output data count. This is problematic for the
buffer's rework as we don't want to rely on this anymore. This commit
modifies these functions to take an offset (relative to the buffer's
head) and a maximum byte count. Thus h2_snd_buf() now calls them with
buf->o and takes care of removing deleted data itself. The send functions
now almost support being passed const buffers (except for the data part
which is still embedded).