Item12900: Implement some session handling improvements

Adds an experimental feature.  If enabled, guests will not have CGI
sessions created.  This will break session variables, strikeone and
editing by guests.

Also implements some improvements to how sessions are created.

git-svn-id: http://svn.foswiki.org/trunk/CommentPlugin@17693 0b4bb1d4-4e5a-0410-9cc4-b2b747904278