avoid zgrep -q as it's failing on some distros

src/lxc/lxc-checkconfig.in

@@ -10,7 +10,7 @@ SETCOLOR_WARNING="printf \\033[1;33m"
 SETCOLOR_NORMAL="printf \\033[0;39m"
 
 is_set() {
-    $GREP -q "$1=[y|m]" $CONFIG
+    $GREP "$1=[y|m]" $CONFIG > /dev/null
     return $?
 }