wgsd-client: Add systemd timer/service example

cmd/wgsd-client/systemd/README.md

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+# wgsd-client systemd Integration
+
+systemd timers can be used to periodically invoke `wgsd-client`.
+
+## Installation
+
+* Copy `wgsd-client@.service` to `/etc/systemd/system/`.
+* Copy `wgsd-client@.timer`  to `/etc/systemd/system/`.
+
+## Configuration
+
+* To configure `wgsd-client` for WireGuard interface `wg-foo` copy `wg-example.template.env` to `/etc/wgsd/wg-foo.env` and adjust variables accordingly.
+
+## Activation
+
+* To activate periodic `wgsd-client` invocation for WireGuard interface `wg-foo` run `systemctl enable wgsd-client@wg-foo.timer`.

cmd/wgsd-client/systemd/etc/systemd/system/wgsd-client@.service

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+[Unit]
+Description=WireGuard Service Discovery (wgsd) Client
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+PermissionsStartOnly=true
+LimitNOFILE=512
+LimitNPROC=16
+CapabilityBoundingSet=CAP_NET_ADMIN
+AmbientCapabilities=CAP_NET_ADMIN
+NoNewPrivileges=true
+User=wgsd-client
+Group=wgsd-client
+DynamicUser=true
+EnvironmentFile=/etc/wgsd/%i.env
+ExecStart=/usr/local/bin/wgsd-client -device %i -dns $DNS -zone $ZONE

cmd/wgsd-client/systemd/etc/systemd/system/wgsd-client@.timer

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+[Unit]
+Description=Run WireGuard Service Discovery (wgsd) Client
+
+[Timer]
+OnActiveSec=10
+OnUnitActiveSec=45
+AccuracySec=1s
+Persistent=true
+
+[Install]
+WantedBy=timers.target

cmd/wgsd-client/systemd/etc/wgsd/wg-example.template.env

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+DNS=10.13.37.1:5353
+ZONE=wg.example.net.