[MEDIUM] Collect & show information about last health check, v3

[MEDIUM] Collect & show information about last health check, v3

Collect information about last health check result,
including L7 code if possible (for example http or smtp
return code) and time took to finish last check.

Health check info is provided on both stats pages (html & csv)
and logged when a server is marked UP or DOWN. Currently active
check are marked with an asterisk, but only in html mode.

Currently there are 14 status codes:
  UNK     -> unknown

  INI     -> initializing
  SOCKERR -> socket error

  L4OK    -> check passed on layer 4, no upper layers testing enabled
  L4TOUT  -> layer 1-4 timeout
  L4CON   -> layer 1-4 connection problem, for example "Connection refused"
              (tcp rst) or "No route to host" (icmp)

  L6OK    -> check passed on layer 6
  L6TOUT  -> layer 6 (SSL) timeout
  L6RSP   -> layer 6 invalid response - protocol error

  L7OK    -> check passed on layer 7
  L7OKC   -> check conditionally passed on layer 7, for example
               404 with disable-on-404
  L7TOUT  -> layer 7 (HTTP/SMTP) timeout
  L7RSP   -> layer 7 invalid response - protocol error
  L7STS   -> layer 7 response error, for example HTTP 5xx