Rework os resolver

Currently, os resolver not only handle A and AAAA records, but also does
it wrongly, since when it packs AAAA record to a dns.A record.

This commit reworks os resolver to make it works with all supported
record types.

nameservers_unix.go

@@ -0,0 +1,23 @@
+//go:build !js && !windows
+
+package ctrld
+
+import (
+	"net"
+
+	"tailscale.com/net/dns/resolvconffile"
+)
+
+const resolvconfPath = "/etc/resolv.conf"
+
+func nameservers() []string {
+	c, err := resolvconffile.ParseFile(resolvconfPath)
+	if err != nil {
+		return nil
+	}
+	ns := make([]string, 0, len(c.Nameservers))
+	for _, nameserver := range c.Nameservers {
+		ns = append(ns, net.JoinHostPort(nameserver.String(), "53"))
+	}
+	return ns
+}