Changing INET6 to PPP_WITH_IPV6CP and adding configure option

Based on feedback on PR #296, the option ipv6-support seems inconsistent
with the existing ipxcp option. Futhermore, the #define has been renamed
to avoid bleeding into third party projects.

pppdconf.h is already distributed and will define or undefine the
PPP_WITH_IPV6CP define.

Signed-off-by: Eivind Næss <eivnaes@yahoo.com>