This commit wraps the `fd` of type `i32` to an `OwnedFd` in order to
ensure that it gets properly dropped in both: success and error cases.

Previously, it was already properly dropped if things got wrong during
and after the `OnionTunnel::create_with_fd` call, but not beforehand.

Now it gets wrapped around an `OwnedFd` in the first step, meaning that
it will be properly closed in all cases.

This commit also adjusts the function prototypes in the `onion-tunnel`
crate respectively in order to accept higher level arguments of file
descriptors.

Fixes #116