Raw file descriptor access

[jimblandy]

If people want to do something other than what we've provided for in the library, then they should be able to get the raw file descriptor out of a Counter or Group; those types should implement IntoRawFd.

Conversely, it should be possible to construct those types from raw file descriptors, although we need to populate the id and max_members fields somehow.

[jimblandy]

All that's left here is construction. I don't think we can use any stock traits for that, so I guess there should just be unsafe methods on those types.