ext/pcntl: fix declared signature of pcntl_signal($restart_syscalls)

[lacatoire]

pcntl_signal() parses its third argument with Z_PARAM_BOOL_OR_NULL(), so null is accepted, and null is also what selects the SIGALRM specific default of false:

if (restart_syscalls_is_null && signo == SIGALRM) {
    restart_syscalls = 0;
}

The stub, however, declared a non-nullable bool defaulting to true, so Reflection reported a signature the implementation does not honour:

$p = (new ReflectionFunction('pcntl_signal'))->getParameters()[2];

var_dump($p->allowsNull());        // bool(false)
var_dump($p->getDefaultValue());   // bool(true)

pcntl_signal(SIGALRM, fn() => null, null);
echo "null accepted\n";            // no TypeError, no deprecation

Every other Z_PARAM_BOOL_OR_NULL() user declares a nullable parameter defaulting to null — imageinterlace(), json_decode(), libxml_use_internal_errors() and sapi_windows_vt100_support(). This aligns pcntl_signal() with them.

Runtime behaviour is unchanged: null was already accepted, and null and an omitted argument already took the same path. What changes is the declared signature, so Reflection now reports ?bool $restart_syscalls = null.

Fixes #23530

[devnexen]

fix is good but you might be able to test it with declare(strict_types=1) ?