- The initial `impl_rearm()` should always be performed by the
worker thread, so that we won't get spurious zero returns from
`watcher_poll()`.
- Sandboxing should not be implicitly disabled if not implemented.
- Shift internal watcher flags, to save space for public ones
if we wish to add any in the future.
- Use negated errno as return value.
- Do not consider deletion of the watched file as a fatal error,
and use a separate error code to distinguish between them.
- Lazy-init worker: Starts watching upon the first call to
watcher_poll().
