Track traffic across interface and device restarts and limit
consumption to a donation-backed amount.

To increase SSD lifetime persistent storage is only touched every
$MaxPerTrafficDiff bytes (currently 100 MB). So tracking can be
trivially avoided by frequent router reboot.

* Usage
  - call traffic_track via cron as often as possible
  - call traffic_account_donation to increase the traffic limit
    after you put the coin in the treasure chest

* Internals
** Counters
   - Max :: traffic limit
   - Per :: traffic accounted to disk
   - Eph :: traffic acounted to /tmp
   - Cur :: traffic directly obtained from `ip -s`
   - Obs :: part of Cur already accounted
** Caveats
  - scripts are non-reentrant