vrrp: add default case in getopt_long

 when starting keepalived with an option without an argument that
 requires an argument keepalived should not be started.

```
$ strace /usr/sbin/keepalived -n -f /etc/keepalived/keepalived.conf --log-facility
execve("/usr/sbin/keepalived", ["/usr/sbin/keepalived", "-n", "-f", "/etc/keepalived/keepalived.conf", "--log-facility"], [/* 24 vars */]) = 0
[...]
write(2, "/usr/sbin/keepalived: option '--"..., 67/usr/sbin/keepalived: option '--log-facility' requires an argument
) = 67
exit_group(0)
```