signal: doesn't wakeup the polling driver #895

The polling crate handles EINTR internally: smol-rs/polling#164

So a simple call to AsyncFlag::notify won't wake up the driver.

There are choices:

• Bring compio_runtime::event back to make it always notify the driver, and use it in compio-signal.
• Always call notify inside a different thread.
• Use pipe instead of AsyncFlag to notify.

cc: @George-Miao