Undo pre-syscall actions in case of failure

For some syscalls DR does complex pre-syscall handling (like clone, clone3,...) in `core/unix/os.c`. If these syscalls fail, we should ensure that we undo pre-syscall stuff if needed.

For clone3, we handle the case where we know it will return `ENOSYS` by detecting whether the system supports clone3 or not during init time, and skipping the pre-syscall stuff if we see a clone3 later. This issue is to track handling of other error scenarios and other such syscalls.