add support for cache directory

make configs use specific directory variables instead of just base_dir
so that one configuration can be used for fhs and legacy installations

src/include/switch_types.h

@@ -510,6 +510,7 @@ struct switch_directories {
 	char *htdocs_dir;
 	char *grammar_dir;
 	char *storage_dir;
+	char *cache_dir;
 	char *recordings_dir;
 	char *sounds_dir;
 	char *lib_dir;