add: Stat polling support while `fs.watch` doesn't work.

Add a new cli option: -g --polling [SPAN]

    If state polling mode is enabled, use it.
    Else use the native api.

    This is useful while watching remote directory.
    Such as the `fs.watch` won't catch the SMB server's file change event.
mov: Change the option name `--polling` to `--watch-polling`, and only leaves the long option name.
opt: Optimize the comment of the `--watch-polling` option.
fix: `opts.polling` changed to `opts[watch-polling]`
Merge pull request #3212 from ysmood/issue3210

add: Stat polling support while `fs.watch` doesn't work.