Aditions

- Add support for `openbsd`, `netbsd`, and `dragonfly`.
- `InsertError<E>` now implements `std::error::Error`.

Changes

- Allow non-`'static` dispatch `Data`. `Data` is passed as an argument to the
  `callback`s while dispatching. This change allows defining `Data` types which
  can hold references to other values.
- `dispatch` now will retry on `EINTR`.