Implement an option to administratively disable the negotiation of

Implement an option to administratively disable the negotiation of
IPv6 on an sppp interface.  In an IPv6-enabled kernel, every IPv6
interface automatically gets an IPv6 address assigned (and IPv6
multicast packets sent at initialization time).  For sppp links where
we know our remote peer wouldn't support IPv6 at all, there's no point
in attempting to negotiate IPV6CP (or to even dial out for an IPv6
packet at all for dial-on-demand interfaces).

I wish there were a more generic way to administratively disable IPv6
on an interface instead.  ume told me there isn't.

While i was at it, converted both, enable_vj and enable_ipv6 into flag
bits in struct sppp (enable_vj used to be an int of its own).

MFC after:	1 month