add configfile path to proctitle

Just a small change to proctitle, showing which configuration file was used to start the server, or when it is using the default configuration.
The results will be like that

```
./redis-server *:6379 /tmp/redis.conf
./redis-server *:6379 [default configuration]
./redis-server *:6379 [cluster] /tmp/redis.conf
./redis-server *:6379 [sentinel] [default configuration]
```