ipv6cp: Fix enforcing local and peer non-random IPv6 interface identifiers (#283)

ipv6cp: Fix enforcing local and peer non-random IPv6 interface identifiers (#283)

In some cases peer may reject our local IPv6 identifier or may send to us
IPV6CP request without any IPv6 identifier or send empty IPv6 identifier
(asking as to generate some identifier for him).

In these special cases pppd always generated some new random IPv6
identifier and completely ignored the fact that user may already specified
IPv6 link local address (used for IPv6 identifier) either at command line
or in config file.

So properly check pppd options and generate new random IPv6 identifier only
in case user did not supply any IPv6 link local address.

If pppd was configured to not allow random identifiers and peer rejected
our enforced identifiers then pppd connection should be terminated.

Signed-off-by: Pali Rohár <pali@kernel.org>