Allow env var AACS_HOME for config home

Allows a user/app to set an env var AACS_HOME for libaacs to use as its user config home location. It allows more granular control without having to rely on HOME that may not be appropriate for an app's control.

The primary use is for Kodi to be able to manage the location and set it to its user accessible data locations other Kodi specific data is stored.

Implementation has been tested on macos with appropriate aacs data located in the $AACS_HOME/aacs path and playback inside Kodi with an appropriately aacs encrypted iso