Add systemd .service file for redshift

Add a systemd user .service file to allow users to run redshift as a
daemon using systemd. Also adds a new configure option,
`--with-systemduserunitdir`, which gives the install path.

If set to `no`, systemd support is disabled.
If not set, we use `pkg-config` to find the path.

data/systemd/redshift.service.in

@@ -0,0 +1,11 @@
+[Unit]
+Description=Redshift display colour temperature adjustment
+Documentation=http://jonls.dk/redshift/
+After=display-manager.service
+
+[Service]
+ExecStart=@bindir@/redshift
+Restart=always
+
+[Install]
+WantedBy=default.target