Improve the fix for bug #19701

Improve the fix for bug #19701

 src/dired.c (directory_files_internal, file_name_completion)
 [WINDOWSNT]: Signal an error when errno is set non-zero by
 'readdir', regardless of its value.
 src/w32.c (sys_readdir): Set errno to ENOENT when the directory
 doesn't exist and to EACCES when it's not accessible to the
 current user.  Set errno to zero when FindNextFile exhausts the
 directory, so that callers don't interpret that as an error and
 don't signal a file-error.
 (open_unc_volume): Set errno to ENOENT if WNetOpenEnum fails.