Emulate "[inet] autoconf" hostname.if(5) lines with "dhcp"

Emulate "[inet] autoconf" hostname.if(5) lines with "dhcp"

With dhcpleased(8) in base, netstart(8) and ifconfig(8) understand both
"autoconf" and "inet autoconf" lines in hostname.if(5) files to signal the
new daemon.

The installer however currently has only dhclient(8), hence manual upgrades
with "[inet] autoconf" instead of "dhcp" in hostname.if files would fail to
establish IPv4 connectivity.

Make install.sub's netstart clone treat autoconf lines like old fashioned
dhcp lines such users^Wearly testers of the new approach don't get stuck in
nyetwork land.

Note that this is only relevant for manual upgrades;  installation always
creates working hostname.if files and automated upgrades with sysupgrade(8)
do not care about network/hostname.if files.

Idea from deraadt
OK deraadt krw ajacoutot