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.