Parameter SOCKET

I suggest an additional parameter "SOCKET" for the config file (`.xtrabackup.config`) defaults to 
`/tmp/mysql.sock`. The reason is the following innobackupex's error: 

```
Failed to connect to MySQL server: Can't connect to local MySQL server 
through socket '/tmp/mysql.sock' (2).
```

My actual socket is `/var/run/mysql/mysql.sock`.
