Review `Inotify::add_watch` and `WatchMask`

[hannobraun]

When adding a watch using Inotify::add_watch, a WatchMask must be passed. This WatchMask defines which events should be watched and how to watch them. These two things, which events to watch for, and the configuration of that watch, could be considered separate concerns.

I'm thinking it may be better to split WatchMask into two parameters, but I'm not sure at this point.