Unix Domain Sockets support

My (fork of) mongo doesn't listen any tcp ports, listens only on unix socket, had to add unix socket support.

Adds `mongo_sock` server configuration variable; 

```php
$MONGO["servers"][$i]["mongo_name"] = "Localhost";  //mongo server name
$MONGO["servers"][$i]["mongo_sock"] = "/var/run/mongo.sock";  //mongo socket path (instead of host and port)
//$MONGO["servers"][$i]["mongo_host"] = "127.0.0.1";  //mongo host
//$MONGO["servers"][$i]["mongo_port"] = "27017";  //mongo port
```

I am note sure if any other parts of the code needs to be changed, but looks like working like this.