net: import linux tap ioctl definitions

Making features dependent on the availability of newer versions if_tun.h
is going to get seriously clumsy, so let's just import the definitions
we need. It's only a small handful.

If and when we're comfortable depending on 2.6.30 headers, we can remove
this again.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>