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

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), but in the
same directory as the executable
Merge branch 'getappdir-linux-bsd' of https://github.com/DanielGibson/jfbuild