Added systemd service file.

**This is a shadowsocks server service, not client service.**
If you need client service, please modify ssserver to sslocal.

For those who wonder why I didn't use the -d start/stop/restart instead
of using forking type of service, shadowsocks defaults writes to a log
file under daemon mode instead of output to stdout/stderr, which can't
be captured by systemd, so I use the simple mode and keep shadowsocks
foreground.

utils/systemd/shadowsocks@.service

@@ -0,0 +1,14 @@
+[Unit]
+Description=Shadowsocks daemon %i
+After=network.target
+
+[Service]
+ExecStart=/usr/local/bin/ssserver -c /etc/shadowsocks/%i.json
+Restart=on-failure
+KillMode=process
+KillSignal=SIGQUIT
+TimeoutStopSec=10
+
+[Install]
+WantedBy=multi-user.target
+