fix pipe2 silently ignoring unknown flags on old kernels

kernels using the fallback have an inherent close-on-exec race
condition and as such support for them is only best-effort anyway.
however, ignoring potential new flags is still very bad behavior.
instead, fail with EINVAL.

src/unistd/pipe2.c

@@ -8,6 +8,7 @@ int pipe2(int fd[2], int flag)
 	if (!flag) return pipe(fd);
 	int ret = __syscall(SYS_pipe2, fd, flag);
 	if (ret != -ENOSYS) return __syscall_ret(ret);
+	if (flag & ~(O_CLOEXEC|O_NONBLOCK)) return __syscall_ret(-EINVAL);
 	ret = pipe(fd);
 	if (ret) return ret;
 	if (flag & O_CLOEXEC) {