Support PHP 8, use Socket object instead of socket resource

[clue]

This changeset adds support for PHP 8 (released just yesterday). PHP 8 uses Socket objects instead of socket resource and has some stricter error reporting semantics, it otherwise works as-is.

I've also had to skip one test which currently causes a SEGFAULT on PHP 8, this should probably be reported upstream:

class Test extends PHPUnit\Framework\TestCase
{
    public function testA()
    {
        $socket = socket_create_listen(0);
        socket_set_nonblock($socket);
        $ret = socket_accept($socket);

        $this->assertFalse($ret);
    }
}

Builds on top of #59 and #60