Adding more source for getting available DNS

On some platforms, the gateway may not be a usable DNS. So extending the
current approach to allow retrieving DNS from many sources.

nameservers.go

@@ -0,0 +1,29 @@
+package ctrld
+
+import "net"
+
+type dnsFn func() []string
+
+func nameservers() []string {
+	var dns []string
+	seen := make(map[string]bool)
+	ch := make(chan []string)
+	fns := dnsFns()
+
+	for _, fn := range fns {
+		go func(fn dnsFn) {
+			ch <- fn()
+		}(fn)
+	}
+	for range fns {
+		for _, ns := range <-ch {
+			if seen[ns] {
+				continue
+			}
+			seen[ns] = true
+			dns = append(dns, net.JoinHostPort(ns, "53"))
+		}
+	}
+
+	return dns
+}