Support for pcntl_async_signals in NativeDriver

[Ekstazi]

Php 7.1 introduce new way to handle signals.

If pcntl_async_signals is set to true then all signals will be processed async and immediate.

But if context of execution is not in NativeDriver (in synchronous code) then this error happens:

^CPHP Warning:  Invalid callback Amp\Loop\NativeDriver::handleSignal, cannot access private method Amp\Loop\NativeDriver::handleSignal() in /app/test.php on line 9
PHP Stack trace:
PHP   1. {main}() /app/test.php:0
PHP   2. Amp\Loop::run($callback = *uninitialized*) /app/test.php:11
PHP   3. Amp\Loop\NativeDriver->run() /app/vendor/amphp/amp/lib/Loop.php:84
PHP   4. Amp\Loop\NativeDriver->tick() /app/vendor/amphp/amp/lib/Loop/Driver.php:72
PHP   5. Amp\Loop\NativeDriver->dispatch($blocking = *uninitialized*) /app/vendor/amphp/amp/lib/Loop/Driver.php:134
PHP   6. {closure:/app/test.php:8-10}(*uninitialized*, *uninitialized*) /app/vendor/amphp/amp/lib/Loop/NativeDriver.php:130

Example code to reproduce error:

<?php
require __DIR__.'/vendor/autoload.php';

pcntl_async_signals(true);
\Amp\Loop::onSignal(SIGINT, function (){
	echo "sigterm\n";
});
\Amp\Loop::repeat(100, function(){
	sleep(1);
});
\Amp\Loop::run();

I understand that amp not support sync code and this error will be only in rare cases.

P.S.: sorry for bad english.

[Ekstazi]

May be need to enable async signals by default ?