Add contrib/tor.service for use with systemd

changes/bug8368

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+  o Minor features:
+    - Add a systemd service file (tor.service) that can be installed by
+      Linux distributions that make use of the systemd init daemon.
+      Fixes bug 8368.

contrib/tor.service

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+[Unit]
+Description = Anonymizing overlay network for TCP
+After = syslog.target network.target nss-lookup.target
+
+[Service]
+Type = simple
+ExecStart = /usr/bin/tor -f /etc/tor/torrc
+ExecReload = /bin/kill -HUP ${MAINPID}
+KillSignal = SIGINT
+TimeoutSec = 30
+Restart = on-failure
+LimitNOFILE = 32768
+
+# Hardening
+PrivateTmp = yes
+DeviceAllow = /dev/null rw
+DeviceAllow = /dev/urandom r
+InaccessibleDirectories = /home
+
+[Install]
+WantedBy = multi-user.target