Adds swap service.

unit-files/swapon.service

+[Unit]
+Description=Turn on swap
+[Service]
+Type=oneshot
+RemainAfterExit=true
+ExecStartPre=-/bin/bash -euxc ' \
+  fallocate -l 8192m /swap &&\
+  chmod 600 /swap &&\
+  mkswap /swap'
+ExecStart=/sbin/swapon /swap
+ExecStop=/sbin/swapoff /swap
+[Install]
+WantedBy=local.target