adding persistent toggle to zend_stack implementation

Hi, I'd like to use the zend_stack stuff for stacked handlers in the new apache_hooks sapi stuff, but right now, it emalloc's everything, which is unacceptable for my usage. I could write my own stack implementation just for the sapi stuf, but thats seems less productive than making zend_stack more universally useable. I've added a persistent attribute to the struct so that it behaves more like zend_hash, added a zend_stack_init_ex which allows setting of this flag, and #define'd zend_stack_init to do a non-persistent zend_stack_init_ex. Patches are attached. If this is somehow undesireable, please let me know and I will just reimplement stacks for sapi. George