store: invoke "osutil.listdir()" via vfs This patch invokes "osutil.listdir()" via vfs object. The function added newly to "abstractvfs" is named not as "listdir()" but as "readdir()", because: - "os.listdir()" seems to be more familiar as "listdir()" than "osutil.listdir()" - "osutil.listdir()" returns also type of each files like "readdir()" POSIX API: even though "d_type" field of "dirent" structure is defined mainly only on BSD/Linux This patch invokes "osutil.listdir()" via "rawvfs" object to avoid filename encoding, because the path passed to "osutil.listdir()" shouldn't be encoded. This patch also omits importing "osutil" module, because it is no longer used.