Change default default qdisc from fq_codel to sch_fq

systemd version the issue has been seen with

https://github.com/systemd/systemd/commit/e6c253e363dee77ef7e5c5f44c4ca55cded3fd47

Used distribution

Ubuntu 18.04

Expected behaviour you didn't see

Better performance than sch_fq

Unexpected behaviour you saw

Packet loss at the sender (ok, not really unexpected)

Steps to reproduce the problem
The main problem is that fq_codel is nice for routers or forwarding devices, but not for end-systems or servers (and I assume that this is rather the majority of linux installations): it doesn't provide an advantage there, but a serious drawback: packet loss at the sending end-system (note that this will also increase the overall latency for the affected TCP connections!). Normally, this should be avoided by using back-pressure mechanisms locally. However, CoDel drops packets instead of propagating back-pressure.
The steps to reproduce the problems are described here:
https://lists.bufferbloat.net/pipermail/bloat/2018-June/008318.html
(BTW: there is no technical argument in this thread for using fq_codel as default).
So if you're using Linux for a software-based router (openwrt), you'll need a special configuration anyway (so it's easy to set fq_codel as default there), but for servers and normal end-systems/hosts the default should be sch_fq, not fq_codel!

Start with the systemd version referenced in the issue and the linked reproduction discussion to understand how the current fq_codel default is selected. Trace the relevant default configuration entry, then verify that normal end systems use sch_fq while preserving any explicitly configured qdisc behavior.