[php-src] PR #23531: ext/pcntl: fix declared signature of pcntl_signal($restart_syscalls)

Pull Request: https://github.com/php/php-src/pull/23531 Author: 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: ```c 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: ```php $p = (new ReflectionFunction('pcntl_signal'))->getParameters()[2]; var_dump($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