Prefix delegation used to be the only supported feature of our DHCPv6
client, but by now it also supports MUD, DNS recursive name servers and
IA_NA is on the horizon. So it makes sense to make IA_PD an optional
module like all those other features are as well.
There is no real reason for that pseudo-module to use the `gnrc_`
prefix. Neither does it need GNRC-components (except, but optionally, as
a network stack of course), nor is it implemented with in the GNRC
network stack.
This implements a client for DHCPv6 IA_PD (Identity Association for
Prefix Delegation). Goal was to have a IETF-compliant alternative to
UHCP. The implementation was based on RFC 8415.
