rrdcached, rrdtool plugins: Honor the "DataDir" setting.

This fixes a regression introduced in:
bfd3f06ef4996707036756371c5724ebf2748821

Github: #380
rrdcached, rrdtool plugins: Add directory separator after datadir.

D'oh! Thanks to @rcpoison for pointing this out!

Github: #380
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>
memcached plugin: actually connect when using a unix socket

Signed-off-by: Florian Forster <octo@collectd.org>
Attempt to fix collectd/collectd#229 by doing user / group lookups before we fork.

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