Unifying DNS from /etc/resolv.conf function

As part of v1.4.0 release, reading DNS from /etc/resolv.conf file is
only available for Macos. However, there's no reason to prevent this
function from working on other *nix systems.

This commit unify the function to *nix, so it could be added as DNS
source for Linux and Freebsd.

nameservers_linux.go

@@ -17,7 +17,7 @@ const (
 )
 
 func dnsFns() []dnsFn {
-	return []dnsFn{dns4, dns6, dnsFromSystemdResolver}
+	return []dnsFn{dnsFromResolvConf, dns4, dns6, dnsFromSystemdResolver}
 }
 
 func dns4() []string {