* add a 'Net Error Check' which treats a missing
  internet availability as an error (disabled by default)
* add a 'List Auto Expiry' which automatically resets
  the 'Faulty Stations' list after n minutes,
  default is '0' which means no expiry (old behaviour).
* rework major parts of the check subroutine
* add both features to LuCI frontend (separate PR/commit)

Signed-off-by: Dirk Brenken <dev@brenken.org>