Fixed directory listing.

The 'ls' command(s) did not show any files for the current directory.
That was because of two reasons:
1. The cookie_data was not initialized properly, next time check the memset() declaration or include 'string.h'.
2. The cookie_data has to be set to zero once the whole directory has been searched, or always stay at the end.

I also removed a warning for out-of-bound integer, all integer constants are assumed to be signed unless post fixed with a capital 'U'. It is a good habit to always post fix unsigned numbers with a capital 'U'.

BR, Vlad