sw-raid: make mdmon start from initrd

Also add required systemd services for starting/stopping mdmon.

Closes #13447.
abbradar: fixed `mdadmShutdown` service name according to de facto conventions.

nixos/modules/system/boot/stage-1.nix

···
 
       # Add RAID mdadm tool.
       copy_bin_and_libs ${pkgs.mdadm}/sbin/mdadm
+      copy_bin_and_libs ${pkgs.mdadm}/sbin/mdmon
 
       # Copy udev.
       copy_bin_and_libs ${udev}/lib/systemd/systemd-udevd

nixos/modules/tasks/swraid.nix

···
     cp -v ${pkgs.mdadm}/lib/udev/rules.d/*.rules $out/
   '';
 
+  systemd.services.mdadmShutdown = {
+    wantedBy = [ "final.target"];
+    after = [ "umount.target" ];
+
+    unitConfig = {
+      DefaultDependencies = false;
+    };
+
+    serviceConfig = {
+      Type = "oneshot";
+      ExecStart = ''${pkgs.mdadm}/bin/mdadm --wait-clean --scan'';
+    };
+  };
+
+  systemd.services."mdmon@" = {
+    description = "MD Metadata Monitor on /dev/%I";
+
+    unitConfig.DefaultDependencies = false;
+
+    serviceConfig = {
+      Type = "forking";
+      Environment = "IMSM_NO_PLATFORM=1";
+      ExecStart = ''${pkgs.mdadm}/bin/mdmon --offroot --takeover %I'';
+      KillMode = "none";
+    };
+  };
+
+  systemd.services."mdadm-grow-continue@" = {
+    description = "Manage MD Reshape on /dev/%I";
+
+    unitConfig.DefaultDependencies = false;
+
+    serviceConfig = {
+      ExecStart = ''${pkgs.mdadm}/bin/mdadm --grow --continue /dev/%I'';
+      StandardInput = "null";
+      StandardOutput = "null";
+      StandardError = "null";
+      KillMode = "none";
+    };
+  };
+ 
 }