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
508f866ea1

nixos/modules/services/computing/slurm/slurm.nix

@@ -303,6 +303,7 @@ in
 
       serviceConfig = {
         Type = "forking";
+        KillMode = "process";
         ExecStart = "${wrappedSlurm}/bin/slurmd";
         PIDFile = "/run/slurmd.pid";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";