Update for session.auto_start:

Update for session.auto_start:
If session is automatically started before the class
definitions are loaded, many functions (compose, certain
parts of read_body, download, etc.) will fail with the
following error:
The script tried to execute a method or access a
property of an incomplete object.

Still to do: ensure that we close the session using
session_write_close() as soon as the page is finished
loading session-specific variables to reduce
contention between frames.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@3717 7612ce4b-ef26-0410-bec9-ea0150e637f0