Fix bug #68163
Fix isset/unset on by-ref names as well
Drop convert_to usage in zend_exceptions

To make everything work correctly with refs.

I'm unsure whether the GET_PROPERTY_SILENT distinction is really
necessary, because the choice seemed pretty random, but kept it
around.
Fix bug #68162: isset($$varname) always true
Merge branch 'master' of git://github.com/php/php-src