Remove the NOINET6 interface flag, a left-over from the times when IPv6

Remove the NOINET6 interface flag, a left-over from the times when IPv6
was enabled by default. Add AFATTACH/AFDETACH ioctls which enable/disable
an address family for an interface (currently used for IPv6 only).

New kernel needs new ifconfig for IPv6 configuration (address assignment
still works with old ifconfig making this easy to cross over).

Committing on behalf of henning@ who is currently lebensmittelvergiftet.
ok stsp, benno, mpi