LRN: two directory patches change the way "is_directory" is evaluated.

LRN: two directory patches change the way "is_directory" is evaluated.
Now it must be GNUNET_YES for the execution flow to switch to a branch
where something is considered to be a directory.
The reason for that is that some functions might return GNUNET_SYSERR
when asked whether something is a directory or not. Checking this
value as "!= GNUNET_NO" will produce positive result, even though it's
not a directory.