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_status to see the different bandwith counters
- 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
