[bp/r50277, r50339 & r50345][SEC] installer:  Replace $_REQUEST with $_POST where possible; Filter input where necessary; require credentials each time installer is entered regardless of session values; Log actions at key points during install

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/6.x@50349 b456876b-0849-0410-b77d-98878d47e9d5