openvpn使用单元文件中的确切ExecStart=调用从命令行开始正常systemd：
/usr/sbin/openvpn --daemon ovpn-server --status /run/openvpn/server.status 10 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/server.conf /run/openvpn/server.pid
ps ax 确认进程在那里：
1634 ? Ss 0:00 /usr/sbin/openvpn --daemon ovpn-server --status /run/openvpn/server.status 10 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/server.conf --writepid /run/openvpn/
但systemctl start openvpn@server没有成功：
? openvpn@server.service - OpenVPN connection to server
Loaded: loaded (/lib/systemd/system/openvpn@.service; disabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Thu 2016-03-17 09:54:52 UTC; 4s ago
Docs: man:openvpn(8)
https://community.openvpn.net/openvpn/wiki/Openvpn23ManPage
https://community.openvpn.net/openvpn/wiki/HOWTO
Process: 1679 ExecStart=/usr/sbin/openvpn --daemon ovpn-server --status …