Add extra latency on top of existing tc qdiscs

On a system with an existing multi-stage qdisc setup, we need to introduce extra latency (at least fixed, but fixed with a small variation would be a nice option to have). The canonical way to do this on Linux is to use the netem qdisc. However, this cannot work here because netem does not work with other qdiscs (this is a [well-documented](https://www.bufferbloat.net/projects/codel/wiki/Best_practices_for_benchmarking_Codel_and_FQ_Codel/#the-netem-qdisc-does-not-work-in-conjunction-with-other-qdiscs) limitation, and one which a coworker has verified himself). Putting a VM onto the machine that does nothing but netem not only seems like overkill but will also massively complicate routing and WLAN configuration so that’s a step I would prefer not to take. In case it’s relevant, the current setup is a combination of htb (used to limit bandwidth only… we probably should have used tbf instead but this is what we had when I joined) and fq_codel for ECN marking, both with custom patches. I’m not averse to patching this into either… As requested, here is a sample setup, using stock htb/fq_codel for easier testing:

#!/bin/mksh
set -ex
dev=eth0
rate=1000
sudo tc qdisc add dev $dev root handle 1: htb default 1
sudo tc class add dev $dev parent 1: classid 1:1 htb rate ${rate}kbit ceil ${rate}kbit prio 1
sudo tc qdisc add dev $dev parent 1:1 handle 2: fq_codel