unix: short-circuit on no-op io watcher changes

Don't add the io watcher to the watcher queue if the requested change
is effectively a no-op, that is, when the event mask doesn't change.

The exception here is sunos because the event ports backend requires
that watched file descriptors are re-added on every turn of the event
loop.

This commit is a micro-optimization, it does not change the event
loop's observable behavior in any way.