[PATCH] - Avoid invoking setitimer when timeouts have been disabled

When compiled with #ifdef ZTS, the setitimer calls in
zend_unset_timeout show up as a performance hotspot even when I'd set
max_execution_time and max_input_time to 0 in php.ini.

I've attached a patch for review that avoids invoking
zend_unset_timeout (and thereby the expensive setitimer call within)
when timeouts have been disabled via settings in php.ini

Thanks,
Arvi