Replace all session_start() calls with sqsession_is_active(). The latter

Replace all session_start() calls with sqsession_is_active(). The latter
is a conditional session_start which will only start a session if none
has been before. This will make us compatible with the upcoming PHP 4.3.3
release. Thanks Phil Driscoll for the tip.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5535 7612ce4b-ef26-0410-bec9-ea0150e637f0