MEDIUM: stats: make stats_dump_be_stats() use stats fields for HTML dump

MEDIUM: stats: make stats_dump_be_stats() use stats fields for HTML dump

Some fields are still needed to complete the conversion :
  - px->srv : used to take decisions when backend has no server (eg: print down or not)
  - algo string (useful for CSV as well) // only if SHLGNDS
  - cookie_name (useful for CSV as well) // only if SHLGNDS
  - px->mode == HTTP (or px->mode as a string) // same for frontend
  - px->be_counters.intercepted_req (stats and redirects ?)

The following field already has a place but was not presented in the
CSV output, so it should simply be added afterwards :
  - px->be_counters.http.cum_req (was in HTML and missing from CSV)