units: use /var/swapfile if found

New unit to automatically make use of a swapfile if
it exists, not a symlink, and /var is writable.

Signed-off-by: Mark D Horn <mark.d.horn@intel.com>

units/meson.build

@@ -153,6 +153,8 @@ units = [
         ['user.slice',                          ''],
         ['var-lib-machines.mount',              'ENABLE_MACHINED',
          'remote-fs.target.wants/ machines.target.wants/'],
+        ['var-swapfile.swap',                   '',
+         'swap.target.wants/'],
 ]
 
 in_units = [

units/var-swapfile.swap

@@ -0,0 +1,19 @@
+#  SPDX-License-Identifier: LGPL-2.1+
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Default Swap File (/var/swapfile)
+Documentation=man:systemd.swap(5)
+ConditionPathExists=/var/swapfile
+ConditionPathIsSymbolicLink=!/var/swapfile
+ConditionPathIsReadWrite=/var/swapfile
+
+[Swap]
+What=/var/swapfile
+Options=discard,fixpgsz