Throw usage error on empty action

src/imapdomo.skel

@@ -92,6 +92,7 @@ main() {
         *)  break ;;
     esac done
     Action="$1"; shift
+    test -n "$Action" || usage -w "no action specified"
     debug -v Action CfgDir LogDir HeaderDir Debug CdTo
     lshandlers | grep -qw "$Action" \
      || die "no handler for action: $Action.lua in $IMAPDOMO_HOME/handlers"