fpm: call zend_signal_init() in child

fpm: call zend_signal_init() in child

This addresses bug #71269.

When an fpm child handles more than one request, zend_signal_startup() will
override the saved signal handlers with the internal zend handlers set from the
previous request, causing a SIGQUIT signal to result in a core dump rather than
gracefully exiting (the expected behaviour).

This is fixed by adding a call to zend_signal_init() after setting the
signal handlers in the child. The same technique is used in the apache SAPI
module in commit fd5a756ad44124ffa7e9a5810a31ff49e91708cf which addresses
bug #61083.