fix: config watcher follow symlinks

[Bubbleioa]

#359

[yujqiao]

This one I'm a bit hesitated. If the config is /etc/rathole.toml, this will result in watching every file change under /etc

[yujqiao]

That said, I cannot think of another way to fix symlink. readlink -f probably will fail when the file is edited by vim because the inode number could change. Can we watch recursively only when the target file is a symlink? so we can limit the impact

[yujqiao]

i don't think the current approach can be merged given comments above