add dir_open_default()

(access temporary directories in host filesystem)

src/file/dir_posix.c

@@ -80,3 +80,8 @@ static BD_DIR_H *_dir_open_posix(const char* dirname)
 }
 
 BD_DIR_H* (*dir_open)(const char* dirname) = _dir_open_posix;
+
+BD_DIR_OPEN dir_open_default(void)
+{
+    return _dir_open_posix;
+}

src/file/dir_win32.c

@@ -104,3 +104,8 @@ static BD_DIR_H *_dir_open_win32(const char* dirname)
 }
 
 BD_DIR_H* (*dir_open)(const char* dirname) = _dir_open_win32;
+
+BD_DIR_OPEN dir_open_default(void)
+{
+    return _dir_open_win32;
+}

src/file/file.h

@@ -59,4 +59,6 @@ BD_PRIVATE BD_FILE_OPEN file_open_default(void);
 
 BD_PRIVATE extern BD_DIR_H* (*dir_open)(const char* dirname);
 
+BD_PRIVATE BD_DIR_OPEN dir_open_default(void);
+
 #endif /* FILE_H_ */