tap-bsd: implement a FreeBSD only version of tap_open
The current behaviour of tap_open for BSD systems differ greatly from
it's Linux counterpart. Since FreeBSD supports interface renaming and
tap device cloning by opening /dev/tap, implement a FreeBSD specific
version of tap_open that behaves like it's Linux counterpart.
This is specially important for toolstacks that use Qemu (like Xen
libxl), in order to have a unified behaviour across suported
platforms.