add systemd startup description

[makefu]

Add a description for systemd to be able to automatically startup pvpn when the system is booting.
The file might look something like that:
/etc/systemd/system/pvpn@.service:

[Unit]
Description=pvpn
After=network.target
[Service]
Type=oneshot
ExecStart=/usr/bin/pvpn -t ssh-3 root@%i default
ExecStop=/usr/bin/pvpn -S
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target

usage: systemctl enable pvpn@127.0.0.1.service

cheers,
makefu