Directory-related functions implemented

* cellFsOpendir, cellFsReaddir, cellFsClosedir functions implemented.
* vfsDirBase: m_entryes, GetEntryes renamed to m_entries, GetEntries
respectively.
* vfsLocalDir: Read() function added to get the entries one by one.
* Moved IsExists() from vfsLocalDir to vfsDirBase to avoid "R6025 pure
virtual function call" error.
* Other minor changes in some functions of sys_fs