tweak the vnetid so it can be optional and therefore cleared/deleted.

tweak the vnetid so it can be optional and therefore cleared/deleted.

the abstract vnetid is promoted to a uin32_t, and adds a SIOCDVNETID
ioctl so it can be cleared.

this is all because i set an assignment on implementing a virtual
network interface and the students got confused when vnetid 0 didnt
show up in ifconfig output.

the vnetid in the vxlan(4) protocol is optional, but the current
code confuses 0 with no vnetid being set. this makes it clear.

ok reyk@ who also simplified my diff