SW: add ndisc6 + rdnssd to GRML_SMALL and GRML_FULL

SW: add ndisc6 + rdnssd to GRML_SMALL and GRML_FULL

It adds only 381kB of additional disk space.

Quoting from the feature request:

| rdnssd is a tool that will listen to DNS server being offered in
| an IPv6 router anouncement and configure the local system to use
| the DNS servers included in there. The daemon just needs to be
| installed and enabled. Without this, an IPv6 only grml system
| will be with network, but without DNS.
|
| While we're at it, IPv6 is much more easily debugged if ndisc6, a
| user space tool to send out IPv6 router solicitations and to
| display the contents of router advertisements received, is
| included.

Thanks: Marc Haber for the suggestion
Closes grml/grml#67