Add 'enable-ipv6' option to pppoe connections, needs PPPd to support

Add enable-ipv6 option to pppoe connections, needs pppd to support ipv6 or this option will break any pppoe connection.
See this commit :
https://github.com/kouak/ppp/commit/481c7120bd08496ae73f8ac96b277fdcf60a55cb