Spawn the DHCPv6 client in EC2 instances via rtsold.

Prior to this commit, EC2 AMIs used a "dual-dhclient" tool which was
launched in place of dhclient and spawned both the base system dhclient
for IPv4 and the ISC dhclient from ports for IPv6.

Now that rtsold supports the "M bit" (managed configuration), we can go
back to having the base system dhclient spawned normally, and provide a
script to rtsold which spawns the ISC dhclient from ports when rtsold
decides that it is appropriate.

Thanks to: bz
MFC after: 1 week
Sponsored by: https://www.patreon.com/cperciva