Aligned systemd service config with the definition in the upstream repo:
https://github.com/Irqbalance/irqbalance/blob/master/misc/irqbalance.service#L7.
Other than adding some level of sandboxing it also fixes the "Daemon
couldn't be bound to the file-based socket." warning reported on
irqbalance startup due to the fact that the "/run/irqbalance" directory
didn't exist. The "RuntimeDirectory" property makes sure it gets
created. The aforementioned warning didn't cause any problems I could
spot though.
I have verified that both `irqbalance` as well as `irqbalance-ui` work
fine with this new systemd service config.
