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

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

Now we avoid directly accessing the proxy and instead we pick the values
from the stats fields. This unveils that only a few fields are missing to
complete the job :
  - know whether or not the checkbox column needs to be displayed. This
    is not directly relevant to the stats but rather to the fact that the
    HTML dump is also a control interface. This doesn't need a field, just
    a function argument.
  - px->mode == HTTP (or px->mode as a string)
  - px->fe_counters.intercepted_req (stats and redirects ?)
  - px->fe_counters.cum_conn
  - px->fe_counters.cps_max
  - px->fe_conn_per_sec

All the last ones make sense in the CSV, so they'll have to be added as well.