update shadowsocks.service

#version2
#通过增加pid参数来实现ss log记录的效果，网上搜索整理并实践的结果，之前版本加-d参数是会报错的，pid指定后则正常启动
#Restart修改为always使其永久生效，参考：https://gist.github.com/guyskk/a9665bc6b2a89b73fae34678b1f6dc6b#gistcomment-2165288
# vi /usr/lib/systemd/system/shadowsocks.service
[Unit]
Description=Shadowsocks Server
After=network.target

[Service]
Type=forking
PIDFile=/run/shadowsocks/ss.pid
PermissionsStartOnly=true
ExecStartPre=/bin/mkdir -p /run/shadowsocks
ExecStart=/usr/bin/ssserver --pid-file /var/run/shadowsocks/ss.pid --log-file /var/log/ss.log -c /etc/shadowsocks.json -d start
Restart=always

[Install]
WantedBy=multi-user.target