Add a C function, opcache_preloading(), that returns true during
preloading. Extensions can use this to detect preloading, not only during
compilation/execution, but also in RINIT()/RSHUTDOWN().
Since opcache currently doesn't install any header, I'm adding a new one:
zend_accelerator_api.h. Header name is based on other files in ext/opcache.
Closes GH-19288
