tap: Use numbered tap/tun devices on all *BSD OS's

The following patch simplifies the *BSD tap/tun code and makes use of numbered
tap/tun interfaces on all *BSD OS's. NetBSD has a patch in their pkgsrc tree
to make use of this feature and DragonFly also supports this as well.

Signed-off-by: Brad Smith <brad@comstyle.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit aa4f082f)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>