More tuned-ppd adjustments

[zacikpa]

This PR brings another round of tuned-ppd adjustments and bug fixes. The individual commit messages explain why the changes are being made.

The most prominent change is dropping reverse profile mapping, i.e., from TuneD to PPD profiles. Instead, the daemon now keeps track of the currently active profile and the so-called base profile. This improves tuned-ppd in two ways:

1. Profile mappings now don't have to be injective.
2. When tuned-ppd is restarted, the initial profile is not the last active profile as inferred from the restored TuneD profile, but the last base profile, as saved into a file by tuned-ppd. The active and the base profile may differ due to profile holds being active before the restart. These should arguably be released when tuned-ppd is restarted.