Say autoconf not dhcp

Say autoconf not dhcp

Do not abuse "dhcp" to say "DHCP and SLAAC".
unwind.conf(5) does so but unwindctl(8) does not;  in fact, the latter
already has `status autoconf' to
    Show nameservers learned from dhclient(8), dhcpleased(8) or slaacd(8).

Adjust unwind's config manual and internal code accordingly;  still accept
the old keyword but do not document it.

hostname.if(5) already advises for `inet[6] autoconf' instead of `dhcp' and
other related daemons don't abuse the word "dhcp" like unwind does.

Feedback sthen
OK florian