Nuke PS(vars), we keep the state of registered session variables now

Nuke PS(vars), we keep the state of registered session variables now
completely in PS(http_session_vars). This avoids bugs which are caused
by a lack of synchronization between the two hashes. We also don't need
to worry about prioritizing one of them.

Add session.bug_compat_42 and session.bug_compat_warn which are enabled
by default. The logic behind bug_compat_42:

IF bug_compat_42 is on, and
IF register_globals is off, and
IF any value of $_SESSION["key"] is NULL, and
IF there is a global variable $key, then
$_SESSION["key"] is set to $key.

The extension emits this warning once per script, unless told otherwise.

"Your script possibly relies on a session side-effect which existed until
PHP 4.2.3. Please be advised that the session extension does not consider
global variables as a source of data, unless register_globals is enabled.
You can disable this functionality and this warning by setting
session.bug_compat_42 or session.bug_compat_warn.