dhclient(8): Add support for IPv6-Only option (RFC 8925)

Accept and validate the ipv6only option. When dhclient receives this
option and IPv6 connectivity is available, stop the DHCP configuration
process and wait for the duration specified by the option before
restarting DHCP discovery.

If the address was previously leased, disassociate it and send a
DHCPRELEASE packet.

Use netlink to check for IPv6 connectivity.

Also, unregister ignored options from default PRL.

Reviewed by: ziaee, kfv
Tested by: Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D56637