Setup: add gentoo support

Setup: add gentoo support

With DHCPv6 we're in a similar boat as with Alpine: The
default templates only ship busybox' udhcpc which doesn't do
ipv6 at all, and if dhclient/dhcpcd are installed there'd
still be no way to configure ONLY dhcpv6 without including
dhcpv4. (We could just dump in a dhclientv6.sh script to
/lib64/netifrc/net/dhclientv6.sh if someone absolutely needs
it...)

Also the fact that the network configuration can in *theory*
be a full blown bash script is a bit inconvenient.