add_entry already checks for list nullity

libbluray explicitely looks for aacs_register_file & aacs_set_fopen,
which need to be exported

Split unit key parsing to unit_key.[hc].
Do not load the file twice (parse it when it is loaded for disc id calculation).
Add error checking.
Check unit key file header.

Useful for testing

Load file from both system and user config dirs
(earlier only one file was loaded).