Some changes related with tracelevels and cleanup

Several small improvements:

* Removing unused Lmt* and unused function
* Removing special treatment for application/x-www-form-urlencoded (I guess that it was still there from the old days in which we weren't able to process application/xml properly with MHD).
* Incomplete payload is no longer a LM_W, but a LM_T to avoid printing always a large chuck of XML in the logs (specially annoying when used in combination with LmtService for input payloads, so we had two large chucks of duplicated XMLs in the log). In addition, note that incomplete payload is a usual situation in HTTP requests, so warning was not probably the best class from a semantic point of view.
* Tracelevel for response output (previously, there was no way of getting response output in the logs)