Add libxml_get_external_entity_loader(), which returns the currently
installed external entity loader, i.e. the value which was passed to
libxml_set_external_entity_loader() or null if no loader was installed
and the default entity loader will be used.
This allows libraries to save and restore the loader, controlling entity
expansion without interfering with the rest of the application.
Add macro Z_PARAM_FUNC_OR_NULL_WITH_ZVAL(). This allows us to get the
zval for a callable parameter without duplicating callable argument
parsing.
The saved zval keeps the object needed for fcc/fci alive, simplifying
memory management.
Fixes#76763.
