Add a systemd service file

This lets us start the doorsd daemon automatically on startup.  We also
configure it to restart if the daemon is ever killed.

Signed-off-by: Workantile <maintainers@workantile.com>

Makefile

@@ -2,6 +2,7 @@
 install:
 	install -v doors.py /usr/bin/doorsd
 	install -D -v doorrc /etc/doorrc
+	install -v doorsd.service /usr/lib/systemd/system/doorsd.service
 
 uninstall:
 	rm -v /usr/bin/doorsd /etc/doorrc

doorsd.service

@@ -0,0 +1,9 @@
+[Unit]
+Description=Workantile Doors RFID Daemon
+
+[Service]
+ExecStart=/usr/bin/doorsd
+Restart=always
+
+[Install]
+WantedBy=multi-user.target