Add option to read param=value pairs from configuration file and add systemd unit to apply configuration at boot

[aedalzotto]

This PR includes the following changes:

• Add upp set -c/--from-conf <file> that reads a configuration file with param=value pairs in each line and applies just as upp set does. An example configuration file could have:

/smc_pptable/FanAcousticLimitRpm=2550
/smc_pptable/FanTargetTemperature=70

• Replace all return 2 by exit(2) to properly signalize parent process with INVALID ARGUMENT return code

• Add a systemd unit to apply pp configurations at boot. This unit is called [email protected] and should be copied to /etc/systemd/system/[email protected] by users or to /usr/lib/systemd/system/[email protected] by package maintainers. This unit should be started (to apply once) or enabled (to apply at boot) with a card instance (as in /sys/class/drm/<card>), e.g. systemctl enable [email protected]. This unit will read the configuration file in /etc/upliftpowerplay/<card>.conf and set the desired parameters.

• Note that the systemd unit sets the DPM to profile_peak before applying upp to avoid the graphics driver crashing due to SMU in sleep mode (see #32 and #37). After applying, it restores to "auto," so users must consider whether a different DPM configuration is applied before the unit is invoked at boot (e.g. in a udev rule).