40network: introduce ip=either6 option

40network: introduce ip=either6 option

In kdump, if dump-target is ssh on ipv6, we need to sync until ipv6 addr
is ready. Currently ip=auto6/dhcp6 provides such function. But in 1st kernel,
it is hard to know whether the ipv6 addr is got by dhcpv6 or SLAAC.
E.g ifcfg-eth* contains DHCPV6C=yes direction, but there is no dhcpv6
server in the network, and then after the system is up, the user
echo 1 > /proc/sys/net/ipv6/conf/eth0/autoconf && accept_ra by manual
to obtain a ipv6 addr. Or vice.
So this patch suggests to make dhcpv6 as auto6 fallback

Signed-off-by: Pingfan Liu <piliu@redhat.com>