EZC: fix crash due to EG(exception) being transmitted between requests

When an exception was thrown from EZC, EG(exception) was left holding a
reference to the smart-allocated exception object, even after the end of
the request. At the end of the request, the object storage was freed by
resetAllocator(), then when zend_wrap_func() was entered in the next
request, EG(exception) would be freed again, causing a crash.

Fixed by installing request event handlers. The requestInit() handler is
analogous to Zend's init_executor().
You mean zend_globals.h before zend.h? It doesn't work:

```
.../hphp/runtime/ext_zend_compat/php-src/Zend/zend_globals_macros.h:61:8: error: ‘ZEND_API’ does not name a type
```

etc.