gnu: slurm: Configure with a "standard" syslogdir.

Until now, we were using syslogdir=$prefix/etc, which did not exist.
Thus, commands would always fail like this:

  $ guix environment --ad-hoc slurm --pure -- squeue -u $USER
  squeue: error: s_p_parse_file: unable to status file /gnu/store/…-slurm-17.11.3/etc/slurm.conf: No such file or directory, retrying in 1sec up to 60sec

* gnu/packages/parallel.scm (slurm)[arguments]: Pass --sysconfdir.