main/ndisc6: move from community
In an ipv6-only slaac setup, there is no DHCP server to provide DNS servers to use. rdnssd, part of ndisc60 is a userspace program that collects this information from router advertisements and updates resolv.conf.
By having this in main, this can be available in the base setup, so that Alpine Linux can be installed in an IPV6-only setup.