bsdinstall: Add new Auto option to netconfig interface selection dialog

This changes the OK / Cancel buttons into Auto / Manual / Cancel, with
Auto being the default. Manual behaves like OK used to, i.e. presents a
series of dialogs asking exactly how to configure the interface, and
Cancel is unchanged, exiting with exit code 1. Auto will attempt to
configure IPv4+DHCP and IPv6+SLAAC with no interaction, failing only if
neither can be configured, thereby supporting all of IPv4-only,
IPv6-only and dual-stack environments. If at least one DNS server is
provided, it will also skip asking for DNS settings, otherwise it will
act like Manual mode for the purposes of DNS settings and prompt. For a
standard dual-stack environment this cuts down the number of netconfig
dialogs from 6 (interface, IPv4, DHCP, IPv6, SLAAC, DNS) to just the
first one.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D43731