darkstat: correct incompatbility with sshd

darkstat: correct incompatbility with sshd
The init file of darkstat is creating the pid in /var/empty and
setting the owner of the directory to darkstat which is incompatible
with sshd as sshd requires /var/empty to be owned by root and not
group or world-writable.  See issue #12420.

This corrects the problem by creating another directory: /var/darkstat
instead without setting the owner, which is not actually required.
Compile tested: not applicable as the changes do not involve
compilation.
Tested on my home router running darkstat.

Signed-off-by: Jean-Michel Lacroix <lacroix@lepine-lacroix.info>