First stab at IPv6-only preferred from RFC8925.

First stab at IPv6-only preferred from RFC8925.

This lets dhcpleased(8) request "IPv6-only preferred". If the
server replies with this option dhcpleased stops and does not request
a lease and deconfigures IPv4 on the interface.

For now this is pretty much useless unless one dynamically configures
pf(4) to act as a CLAT. gelatod(8) from ports can help with this.

However, this helps me while hacking on a kernel based stateless CLAT
by moving dhcpleased out of the way while having an IPv6-mostly
network configured to compare behaviour with macOS.

Input jmc
OK phessler
Input & OK sthen