Ability to dynamically add files to wait for events from

Created by: batonius

Problem: Right now there's no way to add files to wait for events from once EvenQueue loop is going, for example, if a daemon uses non-blocking sockets.

Solution: Add the ability to set the default callback and to subscribe/unsubscribe to events from files via file descriptors. The default callback is responsible for processing the events in the correct way.

Comment: I'm using it in dnsd - https://github.com/batonius/netstack/blob/dnsd/src/dnsd/scheme.rs .