ipv6cp: Add support for ipv6cp-use-remotenumber option

ipv6cp: Add support for ipv6cp-use-remotenumber option

This new option cause that pppd would use "remotenumber" option value for
negotiating IPv6 remote interface identifier.

It is expected that "remotenumber" option in this case is set either to MAC
address, IPv4 address, IPv6 address or telephone number (with or without
plus sign) of remote peer system.

This option is useful for PPPoE connections to generate stable and
predicable IPv6 remote interface identifier as "remotenumber" is set by
pppoe.so plugin to MAC address of remote ethernet peer.

Similarly dial-up connections set "remotenumber" to telephone number of the
remote system and VPN-based ppp plugins set "remotenumber" to address of
remote peer (in case VPN connection is based on IPv4 transport protocol
then address is set to IPv4, if based on IPv6 then remotenumber is IPv6
address).

Having stable IPv6 interface identifiers in ipv6cp is really important.

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