Add support for custom kqueue filters

[notgull]

As suggested here, it may be beneficial to allow support for custom event filters for kqueue-targeting systems, such as EVFILT_SIGNAL for waiting on signals.

Maybe we could have an extension trait API, to the tune of:

pub struct Poller { /* ... */ }

pub mod os {
    #[cfg(uses_kqueue)]
    pub mod kqueue {
        pub trait PollerExt : Sealed {
            fn add_signal(/* ... */);
        }

        impl PollerExt for crate::Poller {
            /* ... */
        }
    }
}