nixos/slurm: set slurmd KillMode to process

The default of systemd is to kill the
the whole cgroup of a service. For slurmd
this means that all running jobs get killed
as well whenever the configuration is updated (and activated).

To avoid this behaviour we set "KillMode=process"
to kill only slurmd on reload. This is how
slurm configures the systemd service.

See:
https://bugs.schedmd.com/show_bug.cgi?id=2095#c24
https://github.com/SchedMD/slurm/commit/508f866ea10e4c359d62d443279198082d587107