use Linux 3.3 netem rate extension instead of tbf qdisc

```
Use the new netem rate extension when available (e.g.: tc qdisc add dev eth0
root netem rate 10kbit) for more accurate emulation results for link bandwidth.

http://blog.jauu.net/linux-network-emulator-extensions.html
http://pf.itd.nrl.navy.mil/pipermail/core-users/2013-December/001456.html

This could be tested using manual link configurations if the bug #128
enhancement (write your own tc netem rules, instead of having CORE generate
them) were available.
```

Original issue reported on code.google.com by `ahrenh...@gmail.com` on 2 Jan 2014 at 3:48
- Blocked on: #128