Add FILESYSTEM_openDirectory() and _openDirectoryEnabled()

Have to add some includes and put these behind some ifdefs, of course.

I'm pretty sure FreeBSD and OpenBSD and Haiku are POSIX enough that the
"open" command will work on them, too.

I would've loved to make FILESYSTEM_openDirectoryEnabled a simple bool
instead of a function, but I ran into issues with putting it in the
FileSystemUtils header file, so I'll just make it a function and call it
a day.

desktop_version/src/FileSystemUtils.h

@@ -20,4 +20,7 @@ bool FILESYSTEM_loadTiXmlDocument(const char *name, TiXmlDocument *doc);
 
 std::vector<std::string> FILESYSTEM_getLevelDirFileNames();
 
+bool FILESYSTEM_openDirectoryEnabled();
+bool FILESYSTEM_openDirectory(const char *dname);
+
 #endif /* FILESYSTEMUTILS_H */