Enforce MW_NO_SESSION, add MW_NO_SESSION_HANDLER

Enforce MW_NO_SESSION, add MW_NO_SESSION_HANDLER

When an entry point specifies MW_NO_SESSION, actually enforce that by
having both SessionManager and PHP's session handling (session_start()
and friends) throw exceptions.

If an entry point needs the old behavior of using PHP's default session
handling (as defined in php.ini), it should define
MW_NO_SESSION_HANDLER instead of or in addition to MW_NO_SESSION.

This also makes PHPSessionHandler be installed in CLI mode, where it
wasn't installed before.

Bug: T127233
Change-Id: I2a3db06ee8e44a044096c57a819b5fd5e51c5c5c