ppp: don't remove addresses from interface while IPCP/IPV6CP is running

pppd also tries to configure addresses by itself through some
ioctls. If we remove between those calls an address that was added,
pppd fails and quits.

To avoid this race condition, don't remove addresses while IPCP and
IPV6CP are running. Once pppd sends an IP configuration, it has
finished configuring the interface and we can proceed normally.

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