ZFS: automatically import zpools and mount zfs filesystems.

Patch contributed by Jack Cummings.

modules/tasks/filesystems/zfs.nix

@@ -63,6 +63,28 @@ in
         '';
     };
 
+    systemd.services."zpool-import" = {
+      description = "Import zpools";
+      after = [ "systemd-udev-settle.service" ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+        ExecStart = "${kernel.zfs}/sbin/zpool import -f -a -d /dev";
+      };
+    };
+
+    systemd.services."zfs-mount" = {
+      description = "Mount zfs volumes";
+      after = [ "zpool-import.service" ];
+      wantedBy = [ "local-fs.target" ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+        ExecStart = "${kernel.zfs}/sbin/zfs mount -a";
+        ExecStop = "${kernel.zfs}/sbin/zfs umount -a";
+      };
+    };
+ 
     system.fsPackages = [ kernel.zfs ];                  # XXX: needed? zfs doesn't have a fsck
     environment.systemPackages = [ kernel.zfs ];
     services.udev.packages = [ kernel.zfs ];             # to hook zvol naming, etc.