Support environment variables in the hrtf_tables config value
Expand environment variables for all config option values when loading
Don't fallback to relative filename handling if opening as absolute fails
Add an explicit cast to make Windows happy
Move OpenDataFile to helpers.c so other sources can use it
Add a note that absolute paths may be used for hrtf_tables
Use OpenDataFile to load soundfonts relative to data directories
Add back the missing PATH_MAX fallback