Rename change_poll_flag to clear_poll_flag

Signed-off-by: LAN Xingcan <lanxingcan@gmail.com>
Fix comment

Signed-off-by: LAN Xingcan <lanxingcan@gmail.com>
Rename handle to os_handle

Signed-off-by: LAN Xingcan <lanxingcan@gmail.com>
Rename handle.{hpp,cpp} => os.{hpp,cpp}

Signed-off-by: LAN Xingcan <lanxingcan@gmail.com>
Move spin::handle::throw_for_last_system_error to spin::

Signed-off-by: LAN Xingcan <lanxingcan@gmail.com>
Use std::system_error

Signed-off-by: LAN Xingcan <lanxingcan@gmail.com>