pppd: Separate IPv6 handling for sifup/sifdown

pppd: Separate IPv6 handling for sifup/sifdown

The current code is buggy regarding handling of link state when using
both IPCP and IPv6CP: if IPv6CP has been set up and if during IPCP
negociation, ipcp_up() fails, it will incorrectly take the interface
down. The simple solution here is to change the platform code to do the
same as on Solaris: separate IPv6CP up/down state handling with sif6up()
and sif6down(), so that we really know when the interface is allowed to
go down.

Signed-off-by: Benjamin Cama <benjamin.cama@telecom-bretagne.eu>
Signed-off-by: Paul Mackerras <paulus@samba.org>