main: Restore profile from kernel configuration

When possible, try to restore the previous profile from the kernel configuration. This makes it easier to force a profile in a startup script, or pick up the same configuration if power-profiles-daemon was restarted for some reason.

Note that this does not save the active profile across reboots (a systemd unit could be used for that if necessary), and that it will likely apply the wrong profile if the daemon crashed when an inhibition or a profile hold were in place.

Closes: #5 (closed)