systemd unit: Rate limit execution

This prevents the udev rule to run autorandr once per monitor added or
removed, instead of once per change -- this is relevant, e.g., if a
notebook is (dis-)connected to/from a docking station.

contrib/systemd/autorandr.service

@@ -1,6 +1,8 @@
 [Unit]
 Description=autorandr execution hook
 After=sleep.target
+StartLimitInterval=5
+StartLimitBurst=1
 
 [Service]
 ExecStart=/usr/bin/autorandr --batch --change --default default