Proper use of graceful restart functionality

[Pavlos Parissis]

Hi all,

I am using latest 2.x version of Bird and I have a question about  graceful restart.

I have  `graceful restart on` for BGP protocol and I currently perform below steps when I want to utilize it:
1. birdc graceful restart
2. systemctl start bird.service

According to the documentation I need `-R` command-line option, thus I have adjusted systemd unit file accordingly:
```
ExecStart=/usr/sbin/bird -R -f -u bird -g bird
```

How does the aforementioned command-line option behave when `birdc graceful restart` is not issued?
I scanned the code to understand it and I think bird is smart enough to know that a graceful restarted was
not requested and it doesn't do anything during the startup. 

While I was testing graceful restart functionality, I noticed that even without the aforementioned command-line
option the upstream peer does not remove the routes until restart timeout expires. So, I was wondering if that
option is real needed thus the question.


Regards,
Pavlos