net/dns: detect when resolvconf points to systemd-resolved.

There are /etc/resolv.conf files out there where resolvconf wrote
the file but pointed to systemd-resolved as the nameserver.
We're better off handling those as systemd-resolved.

> # Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
> #     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
> # 127.0.0.53 is the systemd-resolved stub resolver.
> # run "systemd-resolve --status" to see details about the actual nameservers.

Fixes https://github.com/tailscale/tailscale/issues/3026
Signed-off-by: Denton Gentry <dgentry@tailscale.com>

net/dns/direct.go

@@ -81,11 +81,12 @@ func readResolv(r io.Reader) (config OSConfig, err error) {
 // configuration in bs - one of "resolvconf", "systemd-resolved" or
 // "NetworkManager", or "" if no known owner was found.
 func resolvOwner(bs []byte) string {
+	likely := ""
 	b := bytes.NewBuffer(bs)
 	for {
 		line, err := b.ReadString('\n')
 		if err != nil {
-			return ""
+			return likely
 		}
 		line = strings.TrimSpace(line)
 		if line == "" {
@@ -94,15 +95,15 @@ func resolvOwner(bs []byte) string {
 		if line[0] != '#' {
 			// First non-empty, non-comment line. Assume the owner
 			// isn't hiding further down.
-			return ""
+			return likely
 		}
 
 		if strings.Contains(line, "systemd-resolved") {
-			return "systemd-resolved"
+			likely = "systemd-resolved"
 		} else if strings.Contains(line, "NetworkManager") {
-			return "NetworkManager"
+			likely = "NetworkManager"
 		} else if strings.Contains(line, "resolvconf") {
-			return "resolvconf"
+			likely = "resolvconf"
 		}
 	}
 }