control/controlclient: only use a single DNS label as the hostname.

Fixes #971

Signed-off-by: David Anderson <danderson@tailscale.com>

util/dnsname/dnsname.go

@@ -223,6 +223,14 @@ func NumLabels(hostname string) int {
 	return strings.Count(hostname, ".")
 }
 
+// FirstLabel returns the first DNS label of hostname.
+func FirstLabel(hostname string) string {
+	if i := strings.IndexByte(hostname, '.'); i != -1 {
+		return hostname[:i]
+	}
+	return hostname
+}
+
 var separators = map[byte]bool{
 	' ': true,
 	'.': true,