Reintroduce general xengine extra debugging log level.  This can be controlled at runtime with the command "debug xengine log <level>", as with similar commands.

If log level is 1 then every script load is logged.
This means the <logger name="OpenSim.Region.ScriptEngine.XEngine"> section in OpenSim.exe.config is no longer needed to avoid log spam on regions with many scripts and can be removed.