Stick to `ubuntu:20.04` runners for now, as porting to newer ones implies
fixing a number of compilation warnings which may not be worth the trouble
for an EOL branch.
This makes it possibly to run Linux binaries compiled with udev support on
Linux systems which do not provide udev (typically systemd-less distros).
If udev is missing, we fall back to parsing `/dev/input` like when compiled
without udev support (`udev=no`).
Also adding some verbose debug statements to know which method we're using
when debugging Linux joypad issues.
The libudev so wrappers were generated on Mageia 8 with libudev 246.9 using
https://github.com/hpvb/dynload-wrapper:
```
./generate-wrapper.py --include /usr/include/libudev.h --sys-include '<libudev.h>' \
--soname libudev.so.1 --init-name libudev --omit-prefix gnu_ \
--output-header libudev-so_wrap.h --output-implementation libudev-so_wrap.c
```
(cherry picked from commits a10c259c1d
and e26a1f807b)
Edit: Updated to version 0.2 of dynload-wrapper to fix symbols clobbering as
done in #46143.
It's too hard to get compatibility between GNU and BSD sed,
so let's just use perl oneliners.
And improve it to also remove trailing tabs, not just spaces.
(cherry picked from commit c71e189efd)
Removes AppVeyor and all Travis jobs but the iOS one, which hasn't been ported
to GitHub Actions yet (should be done soon).
Backports new style scripts from `master` branch too to do the same checks.
