Implement function that checks if current is more than x% of max

[fdellwing]

Let's say we want to monitor if the current up or downstream is more than 80% of the total available bandwidth. It would be nice to have a check where I can provide crit and warn value and the app calculates that automatically.

./check_fritz -m downstream_usage -c 80

OK - Current Downstream is 34.47 Mbit/s of 97.46 Mbit/s (35.37% utilization) | perfdata