Allow specifying syslog facility and priority

Hello

As the title says, this commit adds two new configuration directives to the syslog module allowing to specify syslog facility and priority as integer or string.

Here is a configuration example:
```
define module {
    module_name     Syslog
    module_type     syslog
    facility        local3
}
```
Priority would default to info in this case, so it will log to local3.info.