Enable Zend Signals by Default

Hi,

I'm going to enable Zend-Signals subsystem by default.

With "Zend signals" enabled, we would have safe termination signal handling in opcache, that would prevent staying SHM in inconsistent state, in cost of few additional checks on each opcache SHM update (performance impact is negligible).

Zend Signals were originally designed to safely handle execution timeout and other termination signals.

However, termination signals were never handled by OPcache (only by APC).

Also, introduction of "safe execution timeout" handling made a lot of checks in Zend Engine redundant.

Today, I already removed useless checks in ZE, added other checks in OPcahce, and fixed few compatibility problems.

This, actually, didn't change anything with "zend signals" disabled.

https://github.com/php/php-src/pull/1951

So, I propose to switch zend-signals on, and revert back if it makes problems to 7.1 release process.
Any objections?

Thanks. Dmitry.