Add support for pid file

[McLeod095]

It is necessary to add support for the PID file so that it can be used in Preup PostDown wg-quick scripts
for example, in the same shadowsocks there is a flag -f that writes the PID to this file and which can be used when turning off the VPN by executing the command
PostDown=kill -9 $(cat /tmp/shadowsocks.pid)