Preserve original order of nameservers

Preserve original order of nameservers

RFC 2132 "DHCP Options and BOOTP Vendor Extensions"
3.8. Domain Name Server Option says
	Servers SHOULD be listed in order of preference.

tcpdump(8), route(8) monitor and dhcpleasectl(8) -l athn0 show servers from
the DHCP OFFER in their original order, as expected.

resolvd(8) however sorts proposals by priority and IP address before writing
them to resolv.conf(5).

But as the system resolver tries this file's `nameserver' options in the
order appearance, sorting by IP breaks DHCP's intended order and thus may
result in the wrong nameserver being queried.

Sorting by IP is done to later remove duplicates from the file.

Sort by priority alone and ensure uniqueness by iterating over the list of
of proposals and zeroeing duplicates instead to preserve any proposal's
original order.

Spotted on a public wifi OFFERing two local IPs plus 8.8.8.8 in this order
which ended up with 8.8.8.8 being the first entry in my /etc/resolv.conf.

In other words, `route nameserver lo0 2.2.2.2 1.1.1.1 1.1.1.1' now yields
	nameserver 2.2.2.2 # resolvd: lo0
	nameserver 1.1.1.1 # resolvd: lo0
rather than
	nameserver 1.1.1.1 # resolvd: lo0
	nameserver 2.2.2.2 # resolvd: lo0

Feedback OK deraadt