Path handling in pspm_init and pspm_quit

[dominikbach]

Summary

PsPM path handling during startup is opaque. Some folders are put on the path and not removed. It is unclear whether all required folders are put on the path.

1. Collect all folders that need to be put on PsPM path on startup and list them here.
2. In pspm_init check which of these folders are already on the path, and store them in settings.
3. Put all folders on the path.
4. in pspm_quit, remove those folders from the path that were not on the path when PsPM was started.