Rethink Owned FD handling

The is to follow up on the idea shortly discussed in !289 (merged):

Actually now that I think of it, perhaps it'd be much easier if we provided OwnedFd in zvariant so users can deserialize to that if they want Fds to be automatically closed for them and use the existing Fd type if they want to manually do the shutdown themselves for any reason. Then we can just drop disown_fds all together and not have to worry about when to disown fds for users and when not.