Scan bdplus home/system config cache dirs for cached convtabs
If no cached convtab filename was passed via the BDPLUS_CONVTAB env var, find and eventually use a cached convtab located (in this order) in the home or system bdplus config cache dirs, if the cache file contains the discs media key in it's file name.
Restores the cache scan/find functionality contained originally in !2 (closed) - reworked and hopefully without unchecked ptr's or memleaks.