initrd: generate ipv6.method=auto for ip=dhcp6
When a ip=auto6 option is passed to kernel, the old dracut network
module only sets accept_ra in kernel and wait for the address to
appear. Instead, with a ip=dhcp6 option it starts 'dhclient -6',
leaving accept_ra to the initial value (that is already 1). So
ip=dhcp6 in practice does kernel IPv6 autoconf and DHCPv6 at the
same time, without honoring the 'Managed' flag of the router
advertisement.
It seems that the only reason to have distinct auto6 and dhcp6
options was that network module did not support starting DHCPv6 only
when necessary based on the M flag of the RA; so the user had to
specify if DHCPv6 was needed or not.
Given that 1) NM is smarter and can start DHCPv6 only when needed by
RA; 2) DHCPv6 alone only gets a /128 address without a prefix route
and so it's not useful; then it makes sense to generate a connection
with 'ipv6.method=auto' for both ip=auto6 and ip=dhcp6.