[FIX] forum import: Revert r46727 and r49710 - avoid fatal errors by restoring comments lib and class used for list_forums function call; use proper variable for temp folder so that file to be imported can be found; handle null values causing server errors for some fields; improve parsing which was mangling forums posts; keep php line breaks from displaying in the imported forum posts. 
[SEC] forum import: parameterize database insert; change $_REQUEST to $_POST and filter