REORG: stats: functions are now HTTP/CLI agnostic

REORG: stats: functions are now HTTP/CLI agnostic

All the functions that were called "http" or "raw" have been cleaned up
to support either only HTML and use that word in their name, or support
both HTML and CSV and be usable both from the HTTP and the CLI entry
points.

stats_http_dump() now explicitly checks for HTML mode before adding
HTML-specific headers/trailers, and has been renamed since it's now
used by both entry points.

Some more duplicated code could be removed. The patch looks big but it's
mostly due to re-indents resulting from the moves or comments updates.

The calling sequences now look like this :

   cli_io_handler()
       -> stats_dump_sess_to_buffer()      // "show sess"
       -> stats_dump_errors_to_buffer()    // "show errors"
       -> stats_dump_info_to_buffer()      // "show info"
       -> stats_dump_stat_to_buffer()      // "show stat"
          -> stats_dump_csv_header()
          -> stats_dump_proxy_to_buffer()
             -> stats_dump_fe_stats()
             -> stats_dump_li_stats()
             -> stats_dump_sv_stats()
             -> stats_dump_be_stats()

   http_stats_io_handler()
       -> stats_dump_stat_to_buffer()      // same as above, but used for CSV or HTML
          -> stats_dump_csv_header()       // emits the CSV headers (same as above)
          -> stats_dump_html_head()        // emits the HTML headers
          -> stats_dump_html_info()        // emits the equivalent of "show info" at the top
          -> stats_dump_proxy_to_buffer()  // same as above, valid for CSV and HTML
             -> stats_dump_html_px_hdr()
             -> stats_dump_fe_stats()
             -> stats_dump_li_stats()
             -> stats_dump_sv_stats()
             -> stats_dump_be_stats()
             -> stats_dump_html_px_end()
          -> stats_dump_html_end()         // emits HTML trailer