htp: randomization of htp inspection sizes

This is an implementation of #940. It randomize libhtp request
and response size if the same way this has been done for stream
inspection.
htp: display info about randomization

When randomizatin is used display a message about actual values.
When setting final configuration nodes, set the whole tree as final.

Prevents benign log message of parent nodes of final values being
redefined (which ends up having no affect as the final nodes
are protected from being removed).
Use the stack for temporary memory buffers.