Sync with upstream up to revision 76844b29, important fixes:

 - Restore the ability of checking and fixing next free
  cluster in FSINFO.
 - Restore lfcl when LOSTDIR's chain was corrupted and
   overwritten with invalid value.
 - Implement checking of `.' and `..' entries of
   a subdirectory.

Change-Id: Ieddc11136f23a4118a25f539657962786303f911