Return executable's directory in Bgetappdir() on Linux/BSD

f9ed630a0b somehow broke loading any assets for me on Linux (I had them in the same directory as a the executable and I also started the game from that dir so they were in $PWD).

So in case you don't have the .grp file in one of the hardcoded paths (e.g. /usr/share/games/jfsw) or in ~/.jfsw (or Duke equivalent), it didn't work anymore, I "fixed" that by adding the directory the executable lies in to the search path (through Bgetappdir())