I should have probably added this many months ago when I first found out

I should have probably added this many months ago when I first found out
about the $_SESSION/$HTTP_SESSION_VAR not being available until a session
was explicitly started.  99% of the time, this shouldn't be needed, but
this function checks for an active session, and starts one if there isn't
an active one.  This should fix the following references for the $_ vars.

Tested, and doesn't seem to break anything

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5435 7612ce4b-ef26-0410-bec9-ea0150e637f0