Override default ppp options by RADIUS attributes

[marekm72]

Related to #57 but more general, I think it makes sense to allow setting at least some of the PPP options based on RADIUS attributes. Currently they are global variables (such as conf_ipv6) read from the config file [ppp] section, same for all users. These are the defaults, but should be possible to override by RADIUS for individual authenticated users. For example, set ipv6=allow but override with ipv6=deny if no IPv6 related attributes are in Access-Accept. This would allow setting ipv6=deny only for specific customers with buggy Phicomm routers that don't really support IPv6 but try to negotiate it anyway and then fail. In the far future the opposite may be useful - like everyone uses IPv6, some buggy SOHO routers have buggy support for obsolete IPv4 because no one bothered to test it... The closest thing I was able to find in other vendor implementations is described here - https://www.juniper.net/documentation/en_US/junose15.1/topics/concept/aaa-radius-returned-attributes-ipcp-ipv6cp-negotiations-ipv4-ipv6-clients-understanding.html

[marekm72]

That link above is dead, but the idea is similar to already implemented MS-MPPE-Encryption-Policy - the RADIUS atribute can override the default global setting, it would be useful to have something like this for IPv4/IPv6 deny/allow/prefer/require as well.