Security fix for:

Security fix for:

  Internet Explorer bug that allows sends cookies to different sites if
  they have the same ending.  eg moodle.com cookie is sent to smec.moodle.com!!

  Also fixes case where multiple moodle installations may be in different
  directories on the same server.  When you go from one to the other the
  first session will be destroyed now (you can't be logged in the two
  at the same time).  It's just a lot safer this way, for now.

Basically this works by storing the wwwroot variable in the session
and checking it for every page.