workflows: fix ubuntu libsensors pkgname

systemd: put version in service description

CI: switch to Ubuntu 18.04 and 20.04

also simplify by using strategy:matrix:

improve logging behavior

Increase loglevel of non-cyclic message and default loglevel to TF_NFY.
Only thing that stays at TF_INF is the lost sensor message on optional
sensors, so that wouldn't be logged by default any more.

thinkfan-wakeup.service: fix dependencies

the sleep service actually finishes just before going to sleep, so we
have to depend on each sleep service explicitly to make sure the wakeup
script runs *after* waking up.

thinkfan.conf.5: rename complex -> detailed syntax

Hopefully, "detailed" is less off-putting to users than "complex", as
they should really be going for that instead of simple mode.

yamlconfig: replace NULL -> nullptr

include sys/types.h

apparently needed with non-GNU libc's. Fixes github issue #55.

fix degrees char formatting in man page

fixes issue #64, thanks to @mariobl for the patch.

fix watchdog ping

Commit #d395107 didn't fix the whole issue, we also need to subtract the
sleeptime now instead of adding it. Otherwise we'd always ping the
watchdog sleeptime seconds too late.
Thanks to github user @susanwl for spotting this.