Added BBA TAP backend for OS X.

TUN/TAP OS X needs to be installed, and /dev/tap0 chown’d to the user.
The tap0 network interface doesn’t appear until /dev/tap0 is opened by
the program, so all bridging must be done once the game is already
running.

Unfortunately, bridging seems to be broken on OS X, so this does not
actually work over LAN (yet!).