csv plugin: fix forgotten "datadir" in re-implement value_list_to_filename()

In the commit 7f90e30 the re-implementation of value_list_to_filename
forgot to prepend the datadir string.

Signed-off-by: Florian Forster <octo@collectd.org>
csv plugin: set datadir null when free

If you define twice DataDir:

    <Plugin csv>
	DataDir "/opt/collectd/var/lib/collectd/csv"
	DataDir stdout
    </Plugin>

the second time datadir will not be null and
you will get garbage in value_list_to_filename

Signed-off-by: Florian Forster <octo@collectd.org>