Session variables now obey track_vars and gpc_globals settings.

Session variables now obey track_vars and gpc_globals settings.
If track_vars is on then decoded variables appear in $HTTP_STATE_VARS
array. If gpc_globals is on, then session vars are decoded into global
variables. If both are on, then globals and $HTTP_STATE_VARS contents
are references to each other.

The /decoder functions now just need to call php_set_session_var()
this behavior.

@ -Session vars are now decoded into $HTTP_STATE_VARS[] array and the
@ globals, depending on track_vars and gpc_globals settings (Andrei)

# Encoding source is currently only globals. We may want to change this
# in the future.