Small fixes

Hi Ravi,

Here are two small commits to:

* Simplify the ferry() routine in [forked-ferry.cc](https://github.com/ravinet/mahimahi/blob/keithw-simplify/forked-ferry.cc) so it takes only `FileDescriptor` objects and doesn't have to know about `TapDevice` specifically.

* Make sure we don't accidentally assign a `FileDescriptor` to another one (just like the copy constructor, which we already disabled). The reason is that we need `dup()` to properly copy a file descriptor integer; otherwise when we destroy one object it will call `close( fd_ )` and effectively destroy them all.

Cheers,
Keith