Set IFF_BROADCAST and IFF_MULTICAST on vlan interfaces from the

Set IFF_BROADCAST and IFF_MULTICAST on vlan interfaces from the
beginning and simply refuse to attach to a parent without either
flag.

Our network stack cannot handle well IFF_BROADCAST or IFF_MULTICAST
on an interface changing on the fly.  E.g., IP will or won't assign
a broadcast address to an interface and join the all-hosts multicast
group on it depending on its IFF_BROADCAST and IFF_MULTICAST settings.
Should the flags alter later, IP will miss the change and keep using
bogus settings.  This can lead to evil things like supplying an
invalid broadcast address or trying to leave a multicast group that
hasn't been joined.  So just avoid touching the flags since an
interface was created.  This has no practical purpose.

Discussed with:	-net, glebius, oleg
MFC after:	1 week